MOS:LEADELEMENTS
In Wikipedia, the lead section is an introduction to an article and a summary of its most important contents. It is located at the beginning of the article, before the table of contents and the first heading. It is  a news-style lead or "lede" paragraph.

The average Wikipedia visit is a few minutes long. The lead is the first thing most people will read upon arriving at an article, and may be the only portion of the article that they read. It gives the basics in a nutshell and cultivates interest in reading on—though not by teasing the reader or hinting at what follows. It should be written in a clear, accessible style with a neutral point of view.

The lead should stand on its own as a concise overview of the article's topic. It should identify the topic, establish context, explain why the topic is notable, and summarize the most important points, including any prominent controversies. The notability of the article's subject is usually established in the first few sentences. As in the body of the article itself, the emphasis given to material in the lead should roughly reflect its importance to the topic, according to reliable, published sources. Apart from basic facts, significant information should not appear in the lead if it is not covered in the remainder of the article.

As a general rule of thumb, a lead section should contain no more than four well-composed paragraphs and be carefully sourced as appropriate, although it is common for citations to appear in the body and not the lead.

Elements

The lead section may contain optional elements presented in the following order: short description, disambiguation links (dablinks/hatnotes), maintenance tags, infoboxes, foreign character warning boxes, images, navigational boxes (navigational templates), introductory text, and table of contents, moving to the heading of the first section.

Short description is a concise explanation of the scope of the page. See Wikipedia:Short description and Wikipedia:WikiProject Short descriptions for more information.
Disambiguation links should be the first visible elements of the page, before any maintenance tags, infobox, or image; if a reader has reached the wrong page, they will want to know that first. Text-only browsers and screen readers present the page sequentially. A "for topics of the same name ..." disambiguation link is sometimes put at the beginning of an article to link to another article discussing another meaning of the article title. In such cases, the line should be italicized and indented using hatnote templates. Do not make this initial link a section. 
Deletion tags (speedy deletion, proposed deletion, and articles for deletion notices).
Maintenance tags should be below the disambiguation links. These tags inform the reader about the general quality of the article and should be presented to the user before the article itself.
English variety and date style tags help editors maintain consistency in articles as they are developed.
Infoboxes contain summary information or an overview relating to the subject of the article, and therefore should be put before any text (though, in actuality, they will generally appear to the side of the text of the lead). The primary difference between an infobox and a navigational box is the presence of parameters: a navigational box is exactly the same in all articles of the same topic, while an infobox has different contents in each article.
{{Foreign character warning box}} alert readers that the article contains foreign characters which may not be supported by their platform. If required, the warning should be sufficiently near any text using the foreign characters that scrolling is not required to see the warning. This is generally after short infoboxes, but before long ones.
Images. As with all images, but particularly the lead, the image used should be relevant and technically well-produced. It is also common for the lead image to be representative because it provides a visual association for the topic, and allow readers to quickly assess if they have arrived at the right page. Image captions are part of the article text. If the article has disambiguation links (dablinks), then the introductory image should appear just before the introductory text. Otherwise a screen reader would first read the image's caption, which is part of the article's contents, then "jump" outside the article to read the dablink, and then return to the lead section, which is an illogical sequence. See also .
Sidebars are a cohesive collection of links used in multiple related articles to facilitate navigation between those articles. The placement of a sidebar in the lead is generally discouraged; it may be included on a case-by-case basis, placed preferably  the lead-section image and infobox. Outside the lead, sidebars are often placed at the top or bottom of any section of an article. 
All but the shortest articles should start with introductory text (the "lead"), which establishes significance, includes mention of significant criticism or controversies, and make readers want to learn more. The lead has no heading; its length should be commensurate with that of the article, but is normally no more than four paragraphs. See also .
The table of contents (ToC) automatically appears on pages with at least four headings. Avoid floating the ToC if possible, as it breaks the standard look of pages. If you must use a floated TOC, put it below the lead section in the wiki markup for consistency. Users of screen readers expect the table of contents to follow the introductory text; they will also miss any text placed between the TOC and the first heading.

Citations

The lead must conform to verifiability, biographies of living persons, and other policies. The verifiability policy advises that material that is challenged or likely to be challenged, and direct quotations, should be supported by an inline citation. Any statements about living persons that are challenged or likely to be challenged must have an inline citation every time they are mentioned, including within the lead.

Because the lead will usually repeat information that is in the body, editors should balance the desire to avoid redundant citations in the lead with the desire to aid readers in locating sources for challengeable material. Leads are usually less specific than the body, and information in the lead section of non-controversial subjects is less likely to be challenged and less likely to require a source; there is not, however, an exception to citation requirements specific to leads. The necessity for citations in a lead should be determined on a case-by-case basis by editorial consensus. Complex, current, or controversial subjects may require many citations; others, few or none. The presence of citations in the introduction is neither required in every article nor prohibited in any article.

As editors are often unaware of this guideline, good faith should always be assumed when  tags are erroneously added to lead sections.  can be added to article leads that often attract  tags.

Content

Provide an accessible overview

 The lead section should briefly summarize the most important points covered in an article in such a way that it can stand on its own as a concise version of the article. The reason for a topic's noteworthiness should be established, or at least introduced, in the lead (but not by using subjective "peacock terms" such as "acclaimed" or "award-winning" or "hit"). It is even more important here than in the rest of the article that the text be accessible. Editors should avoid lengthy paragraphs and overly specific descriptionsgreater detail is saved for the body of the article. Consideration should be given to creating interest in the article, but do not hint at startling facts without describing them. 

Make the lead section accessible to as broad an audience as possible. In general, introduce useful abbreviations but avoid difficult-to-understand terminology, symbols, mathematical equations and formulas. Where uncommon terms are essential, they should be placed in context, linked and briefly defined. The subject should be placed in a context familiar to a normal reader. For example, it is better to describe the location of a town with reference to an area or larger place than with coordinates. Readers should not be dropped into the middle of the subject from the first word; they should be eased into it.

Relative emphasis

According to the policy on due weight, emphasis given to material should reflect its relative importance to the subject, according to published reliable sources. This is true for both the lead and the body of the article. If there is a difference in emphasis between the two, editors should seek to resolve the discrepancy.

Significant information should not appear in the lead, apart from basic facts, if it is not covered in the remainder of the article, although not everything in the lead must be repeated in the body of the text. Exceptions include specific facts such as quotations, examples, birth dates, taxonomic names, case numbers, and titles. This admonition should not be taken as a reason to exclude information from the lead, but rather to harmonize coverage in the lead with material in the body of the article.

Opening paragraph

The first paragraph should define or identify the topic with a neutral point of view, but without being too specific. It should establish the context in which the topic is being considered by supplying the set of circumstances or facts that surround it. If appropriate, it should give the location and time. It should also establish the boundaries of the topic; for example, the lead for the article List of environmental issues succinctly states that the list covers "harmful aspects of human activity on the biophysical environment".

First sentence

The first sentence should introduce the topic, and tell the nonspecialist reader what or who the subject is, and often when or where. It should be in plain English. 

Do not overload the first sentence by describing everything notable about the subject; instead, spread the relevant information out over the entire lead. Avoid cluttering the first sentence with a long parenthesis containing alternative spellings, pronunciations, etc., which can make the sentence difficult to actually read; this information should be placed elsewhere. 

If possible, the page title should be the subject of the first sentence, although there are exceptions:
 If the article title is merely descriptive—such as Electrical characteristics of dynamic loudspeakers—the title does not need to appear verbatim in the main text.
Similarly, if the page is a list, do  introduce the list as "This is a list of X" or "This list of Xs...". A clearer and more informative introduction to the list is better than verbatim repetition of the title. A good example of this is the List of Benet Academy alumni. (See also Format of the first sentence below.)
When the page title is used as the subject of the first sentence, it may appear in a slightly different form, and it may include variations, including plural forms (particularly if they are unusual or confusing) or synonyms.
If the title has a parenthetical disambiguator, such as Mercury (planet), the parenthetical should be omitted in the text.
Date(s) and/or location(s) should be included in the first sentence if they help the reader to quickly determine if they're at the right article. For instance, in the article Spanish–American War, the text of the lead begins:

If its subject is definable, then the first sentence should give a concise definition: where possible, one that puts the article in context for the nonspecialist. Similarly, if the title is a specialized term, provide the context as early as possible.

 Avoid constructions like "[Subject] refers to..." or "...is a word for..." the article is about the subject, not a term  the subject. For articles that are actually about terms, italicize the term to indicate the use–mention distinction.
For topics notable for only one reason, this reason should usually be given in the first sentence.
If the article is about a fictional character or place, make this clear.

Format of the first sentence

If an article's title is a formal or widely accepted name for the subject, display it in bold as early as possible in the first sentence:

Otherwise, include the title if it can be accommodated in a natural way:

Bolding of title and alternative names

Only the first occurrence of the title and significant alternative names (which should usually also redirect to the article) are placed in bold:

Common abbreviations (in parentheses after the corresponding title) are considered significant alternative names in this sense:

If an article is about an event involving a subject about which there is no main article, especially if the article is the target of a redirect, the subject should be in bold:

Avoid redundancy

Keep redundancy to a minimum in the first sentence. Use the first sentence of the article to provide relevant information that is  already given by the title of the article.
The title need not appear verbatim in the lead if it is descriptive.
For example:

The statement relations are the relations does not help a reader who does not know the meaning of diplomatic relations. The second version sensibly includes new information (that relations were established in 1947) in the first sentence, rather than repeating the title.

If the article's title does not lend itself to being used easily and naturally in the first sentence, the wording should not be distorted in an effort to include it. Instead, simply describe the subject in normal English, avoiding unnecessary redundancy.

Sometimes a little redundancy is unavoidable. For example, the Oxford English Dictionary (OED) has to be called by its proper name in its article, and cannot be called anything other than a dictionary in the first sentence. Even in these cases, the first sentence  provide information not given in the title. But try to rephrase whenever possible. Instead of:

consider:

Both contain some redundancy, but the second is better because it tells us that the OED is the world's most respected dictionary of English.

Avoid these other common mistakes

Links should not be placed in the boldface reiteration of the title in the first sentence of a lead:

As an exception, disambiguation pages may use bolding for the link to the primary topic, if there is one.

In general, if the article's title (or a significant alternative title) is absent from the first sentence, do not apply the bold style to related text that  appear (examples from 1999 Nepalese general election):

Proper names and titles
If the title of the page is normally italicized (for example, a work of art, literature, album, or ship) then its first mention should be both bold and italic text:

If the mention of the article's title is surrounded by quotation marks, the title should be bold but the quotation marks should not be:

Pronunciation

If the name of the article has a pronunciation that is not apparent from its spelling, include its pronunciation in parentheses after the first occurrence of the name. Most such terms are foreign words or phrases (mate, coup d'état), proper nouns (Ralph Fiennes, Tuolumne River, Tao Te Ching), or very unusual English words (synecdoche, atlatl). It is preferable to move pronunciation guides to a footnote or elsewhere in the article if they would otherwise clutter the first sentence.

Do not include pronunciations for names of foreign countries whose pronunciations are well known in English (France, Poland). Do not include them for common English words, even if their pronunciations are counterintuitive for learners (laughter, sword). If the name of the article is more than one word, include pronunciation only for the words that need it unless all are foreign (all of Jean van Heijenoort but only Cholmondeley in Thomas P. G. Cholmondeley). A fuller discussion of pronunciation can come later in the article.

Contextual links

The first sentence should provide links to the broader or more elementary topics that are important to the article's topic or place it into the context where it is notable.

For example, an article about a building or location should include a link to the broader geographical area of which it is a part.

In an article about a technical or jargon term, the first sentence or paragraph should normally contain a link to the field of study that the term comes from.

The first sentence of an article about a person should link to the page or pages about the topic where the person achieved prominence.

Exactly what provides the context needed to understand a given topic varies greatly from topic to topic.

Do not, however, add contextual links that don't relate directly to the topic's definition or reason for notability. For example, Van Cliburn's first sentence links to Cold War because his fame came partly from his Tchaikovsky Competition victory being used as a Cold War symbol. The first sentence of a page about someone who rose to fame in the 1950s for reasons unrelated to the Cold War should not mention the Cold War at all, even though the Cold War is part of the broader historical context of that person's life. By the same token, do not link to years unless the year has some special salience to the topic.

Most Featured Articles contain about 12 to 25 links in the lead, with an average of about 1.5 links per sentence or one link for every 16 words.

Links appearing ahead of the bolded term distract from the topic if not necessary to establish context, and should be omitted even if they might be appropriate elsewhere in the text. For example, a person's title or office, such as colonel, naturally appears ahead of their name, but the word "Colonel" should not have a link, since it doesn't establish context. (Do not, however, reword a sentence awkwardly just to keep a needed contextual link from getting ahead of the bolded term.)

Organisms

When a common (vernacular) name is used as the article title, the boldfaced common name is followed by the italic boldfaced scientific name in round parentheses in the first sentence of the lead. Alternative names should be mentioned and reliably sourced in the text where applicable, with bold type in the lead if they are in wide use, or elsewhere in the article (with or without the bold type, per editorial discretion) if they are less used. It is not necessary to include non-English common names, unless they are also commonly used in English, e.g. regionally; if included, they should be italicized as non-English.

When the article title is the scientific name, reverse the order of the scientific and common name(s) (if any of the latter are given), and boldface as well as italicize the scientific name. Avoid putting the most common name in parentheses (this will suppress its display in some views of Wikipedia, including Wikipedia:Pop-ups and Google Knowledge Graph).

Scope of article
In some cases the definition of the article topic in the opening paragraph may be insufficient to fully constrain the scope of the article. In particular, it may be necessary to identify material that is  within scope. For instance, the article on fever  notes that an elevated core body temperature due to hyperthermia is not within scope. These explanations may best be done at the end of the lead to avoid cluttering and confusing the first paragraph. This information and other meta material in the lead is not expected to appear in the body of the article.

Biographies

A summary of the key points in the main guideline on this:
 Reliably sourced material about encyclopedically relevant controversies is neither suppressed in the lead nor allowed to overwhelm; the lead must correctly summarize the article as a whole.
 Recent events affecting a subject are kept in historical perspective; most recent is not necessarily most notable. Balance new information with old, giving all information due weight.
 Wikipedia is not a memorial site; when a subject dies, the lead should not radically change, nor dwell on the death.
 Do not use primary sources for private details about living persons, including birth dates.

Biographies' first sentence

Under the main guideline on this, the opening paragraph of a biographical article should neutrally describe the person, provide context, establish notability and explain why the person is notable, and reflect the balance of reliable sources.

The first sentence should usually state:
Name(s) and title(s), if any . Handling of the subject's name is covered under MOS:NAMES.
Dates of birth and death (if found in secondary sourcesdo not use primary sources for birth dates of living persons or other private details about them). If specific day–month–year dates for birth/death are given elsewhere in the article, then a simple yearyear range may be sufficient to provide context.
Context (location, nationality, etc.) for the activities that made the person notable.
One, or possibly more, noteworthy positions, activities, or roles that the person held, avoiding subjective or contentious terms. 
The main reason the person is notable (key accomplishment, record, etc.)

Examples:

However, try to not overload the first sentence by describing everything notable about the subject; instead, spread relevant information over the lead section.

Alternative biographical names

The basic instructions for biographical names are summarized below; the main guideline on this provides additional detail.

 While a commonly recognizable form of name will be used as the title of biographical articles, fuller forms of name may be used in the introduction to the lead. For instance, in the article Paul McCartney, the text of the lead begins: " ...".
 The name of a person is presented in full if known, including any given names that were abbreviated or omitted in the article's title. For example, the article on Calvin Coolidge gives his name as 
 If a person changed their full name at some point after birth, the birth name may be given as well, if relevant. For example, a lead may mention a woman's birth name when there are relevant matters before she became notable under her married name; also the lead of the article on Caitlyn Jenner includes her birth name  because she was also notable under that name.
 If a hypocorism (diminutive) that is common in English is often used for the subject in lieu of a given name, it is  inserted into the name or given after it, e.g., Tom Hopper has simply .
 Also acceptable are formulations like , when applicable. Hypocorisms are  put in quotation marks.
 If a person is commonly known by a nickname (other than a hypocorism), it is presented between quote marks following the last given name or initial, as for Bunny Berigan, which has . The quotation marks are not boldfaced.

Alternative names

By the design of Wikipedia's software, an article can have only one title. When this title is a name, significant alternative names for the topic should be mentioned in the article. These may include alternative spellings, longer or shorter forms, historical names, and significant names in other languages. Indeed, alternative names can be used in article text in contexts where they are more appropriate than the name used as the title of the article. For example, the city now called "Gdańsk" can be referred to as "Danzig" in suitable historical contexts. 

The editor needs to balance the desire to maximize the information available to the reader with the need to maintain readability. Use this principle to decide  whether mentioning alternative names in the first sentence, elsewhere in the article, or not at all. 

The title can be followed in the first sentence by one or two alternative names in parentheses.

Separate section usage
If there are three or more alternative names, they should not be included in the first sentence as this creates clutter. Instead, the names may be footnoted, or moved elsewhere in the article such as in a "Names" or "Etymology" section. As an exception, a local official name different from a widely accepted English name should be retained in the lead.

Archaic names, including names used before the standardization of English orthography should be clearly marked as such, i.e., (archaic: name), and should not be placed in the first sentence.

Foreign language

Although Wikipedia's naming conventions recommend the use of English, there are instances where the subject of an article is best known in English-language sources by its non-English name. In this case, the non-English title may be appropriate for the article.  foreign-language names, such as those of people who do not write their names in English, are encouraged.

If the subject of the article is closely associated with a non-English language, a single foreign language equivalent name may be included in the lead sentence, usually in parentheses. For example, an article about a location in a non-English-speaking country will typically include the local-language equivalent:

Do not include foreign equivalents in the text of the lead sentence for alternative names or for particularly lengthy names, as this clutters the lead sentence and impairs readability. Do not include foreign equivalents in the lead sentence just to show etymology. Foreign-language names should be moved to a footnote or elsewhere in the article if they would otherwise clutter the first sentence.

Separate languages should be divided by semicolons; romanizations of non-Latin scripts, by commas. Do not boldface foreign names not normally used in English. Some foreign terms should be italicized. These cases are described in the Manual of Style for text formatting.

Stubs

Where the article is a stub and has no section headings, a lead may not be necessary. Although Wikipedia encourages expanding stubs, this may be impossible if reliably sourced information is not available. Once an article has been sufficiently expanded, generally to around 400 or 500 words, editors should consider introducing section headings and removing the stub classification. Note that as the lead is recommended to be one to four paragraphs in length, articles consisting of up to four full paragraphs usually do not need a lead.

Length

The appropriate length of the lead section depends on the total length of the article. As a general guideline—but not absolute rule—the lead should usually be no longer than four paragraphs. The length of the lead should conform to readers' expectations of a short, but useful and complete, summary of the topic. A lead that is too short leaves the reader unsatisfied; a lead that is too long is intimidating, difficult to read, and may cause the reader to lose interest halfway. The following suggestions about lead length may be useful ("article length" refers to readable prose size):

Most Featured Articles have a lead length of about three paragraphs, containing 10 to 15 sentences, or about 300 words total.

Lead sections that reflect or expand on sections in other articles are discussed at Summary style. Journalistic conventions for lead sections are discussed at News style.

Editing the lead section
All users can edit the lead by clicking the edit link of the whole article. By default, there is no edit link just for the lead section, but registered users can get it by enabling one or both of the following preferences (both require JavaScript):

Comparison to the news-style lead

Wikipedia leads are not written in news style. Although there are some similarities, such as putting the most important information first and making it possible for any reader to understand the subject even if they only read the lead, there are some differences. The lead paragraph (sometimes spelled "lede") of newspaper journalism is a compressed summary of only the most important facts about a story. These basic facts are sometimes referred to as the "five Ws": who, what, when, where, and why. Journalistic leads normally are only one or two sentences long. By contrast, in Wikipedia articles, the first sentence is usually a definition, the lead is longer, and it ultimately provides more information, as its purpose is to summarize the article, not just introduce it.

Cleanup
For a list of template messages related to the clean-up of lead sections, see . Editors are encouraged to improve leads rather than simply tag them.

See also
Help:Section
Wikipedia:How to create and manage a good lead section
Wikipedia:Lead section TT first sentence content
Wikipedia:Lead section TT first sentence format
Wikipedia:Lead section TT text
Wikipedia:Manual of Style/Accessibility
Wikipedia's Good definition policy
Wikipedia:WikiProject Lead Improvement Team

Notes

Special explanatory note

References

Wikipedia Manual of Style (layout)