Irish orthography is very etymological, which allows the same written form to represent all dialects of Irish and remain regular. For example, ("tree") is read in Mayo and Ulster, in Galway, or in Munster. A spelling reform in the mid-20th century lead to , the modern standard written form used by the Government of Ireland, which regulates both spelling and grammar. The reform removed inter-dialectal silent letters, simplified some letter sequences, and modernised archaic spellings to reflect modern pronunciation but it also removed letters pronounced in one dialect but not in another. Some words may have dialectal pronunciations not reflected by their standard spelling, they may have dialectal spellings to reflect this. Alphabet Latin script has been the writing system used to write Irish since the 5th century, when it replaced Ogham which was used to write Primitive Irish and Old Irish. Prior the mid-20th century Gaelic type () was the main typeface used to write Irish, now it is usually replaced by Roman type (). The use of Ogham and Gaelic type today is restricted to decorative or self-consciously traditional contexts. The dot above a lenited letter in Gaelic type is usually replaced by a following in Roman type (e.g. → ). Letters and letter names The traditional Irish alphabet () consists of 18 letters: . It does not contain , although they are used in scientific terminology and modern loanwords/words of foreign origin. occurs in a small number of (mainly onomatopoeic) native words (e.g. "to quack" and "caw") and colloquialisms (e.g. for "chirp" and for "screw"). , when not prefixed to a word initial vowel to show aspiration or after a consonant to show lenition, primarily occurs word initially in loanwords, e.g. "hat". is the only letter not listed by Ó Dónaill. Vowels may be accented with an acute accent (; see below). Accented letters are considered variants of their unaccented equivalent so they are ignored for purposes of alphabetisation, they follow their unaccented equivalents in dictionaries (i.e. , , , ...). English letter names are generally used in colloquial and formal speech but there are modern Irish letter names (based on the original Latin names), similar to other languages that use a Latin script alphabet. Tree names were historically used to name the letters. Tradition taught that they all derived from the names of Ogham letters, though it is now known that only some of the earliest