page_title: Abbreviation
section_title: Periods (full stops) and spaces
hierarchical_section_title: Abbreviation / Style conventions in English / Periods (full stops) and spaces
caption_attribution_description: Tile sign in New York’s 34th Street/Penn Station subway station, showing direction to Pennsylvania Station. Notable for punctuation of abbreviation “Penna.”, using a period even though this only omits internal letters.
context_page_description: An abbreviation is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full version of the word or phrase; for example, the word abbreviation can itself be represented by the abbreviation abbr., abbrv., or abbrev.; NBM, for nil by mouth is an abbreviated medical instruction. It may also consist of initials only, a mixture of initials and words, or words or letters representing words in another language. Some types of abbreviations are acronyms, initialisms, or grammatical contractions or crasis.
  An abbreviation is a shortening by any of these, or other, methods.
context_section_description: 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 No. (an abbreviation of Numero, Number) (to avoid the appearance of "No" (yes and 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). (AMA style also forgoes italic on terms long since naturalized into English from Latin, New Latin, other languages, or ISV; thus, no italic for eg, ie, vs, et al., in vivo, in vitro, or in situ.)
  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 words in the same phrase, so one almost never encounters "U. S."
  When an abbreviation appears at the end of a sentence, only one period is used: The capital of the United States is Washington, D.C.
caption: Sign in New York City subway, reading “Penna.” for Pennsylvania, showing American style of including the period even for contractions.
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