Tardis:Period

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Periods or full stops are generally well understood by most editors, worldwide. However, periods aren't used after Mr, Mrs, or St, nor are they used with acronyms like UNIT, NASA, and CIA

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T:PERIOD

Periods are naturally used at the end of sentences to bring closure. However, because of varied use across the English speaking world, the use of periods with abbreviations must be arbitrarily decided her on our wiki.

In keeping with our general preference for British English usage, the following is "correct" here:

• As mentioned at T:HONOUR, no period shall be used if the first and last letters of a personal title are used to form the abbreviation of that title. A period shall be used if the usual abbreviation does not include these letters. Hence, it's Mr Smith but Rev. Arnold Golightly.
  • without a period: Mr, Mrs, Ms, St
  • with a period: Rev., Sen., Rep., Prof.
• In the use of acronyms, no period shall be used, owing to the modern British standard which has essentially caved into American usage. Yes, UNIT was once U.N.I.T., but nowadays it's not. Hence NASA, CIA, FBI, etc.

You should put no more than two spaces after a period, as is explained elsewhere.