is taken, then all letters should be capitalized, as in YTD for year-to-date, PCB for printed circuit board and FYI for for your information. However, see the following section regarding abbreviations that have become common vocabulary: these are no longer written with capital letters. Periods (full stops) and spaces A period (full stop) is often used to signify an abbreviation, but opinion is divided as to when and if this should happen. According to Hart's Rules, the traditional rule is that abbreviations (in the narrow sense that includes only words with the ending, and not the middle, dropped) terminate with a full stop, whereas contractions (in the sense of words missing a middle part) do not, but there are exceptions. Fowler's Modern English Usage says full stops are used to mark both abbreviations and contractions, but recommends against this practice: advising them only for abbreviations and lower-case initialisms and not for upper-case initialisms and contractions. In American English, the period is usually included regardless of whether or not it is a contraction, e.g. Dr. or Mrs.. In some cases, periods are optional, as in either US or U.S. for United States, EU or E.U. for European Union, and UN or U.N. for United Nations. There are some house styles, however—American ones included—that remove the periods from almost all abbreviations. For example: The U.S. Manual on Uniform Traffic Control Devices advises that periods should not be used with abbreviations on road signs, except for cardinal directions as part of a destination name. (For example, "Northwest Blvd", "W. Jefferson", and "PED XING" all follow this recommendation.) AMA style, used in many medical journals, uses no periods in abbreviations or acronyms, with almost no exceptions. Thus eg, ie, vs, et al, Dr, Mr, MRI, ICU, and hundreds of others contain no periods. The only exceptions are (an abbreviation of Numero, Number), to avoid confusion with the word "No"; initials within persons' names (such as "George R. Smith"); and "St." within persons' names when the person prefers it (such as "Emily R. St. Clair") (but not in city names such as St Louis or St Paul). Acronyms that were originally capitalized (with or without periods) but have since entered the vocabulary as generic words are no longer written with capital letters nor with any periods. Examples are sonar, radar, lidar, laser, snafu, and scuba. Today, spaces are generally not used between single-letter abbreviations of