A reader wonders,
Why on earth do we place a period after Ms? It’s not an abbreviation of anything I know of.
Americans place a period after Ms. because style guides like The Chicago Manual of Style and The AP Stylebook tell us to. British speakers do not place a period (full stop) after Ms because their style guides tell them not to.
American Style Guides
The Chicago Manual of Style
Use periods with abbreviations that end in a lowercase letter: p. (page), vol., e.g., i.e., etc., a.k.a., a.m., p.m., Ms., Dr., et al.
Ms. This is the spelling and punctuation for all uses of the courtesy title, including direct quotations. There is no plural.
British Style Guides
The Penguin Guide to Punctuation
Abbreviations are very rarely used in formal writing. Almost the only ones which are frequently used are the abbreviations for certain common titles, when these are used with someone’s name: Mr Willis, Dr Livingstone, Mrs Thatcher, Ms Harmon, St Joan. (Note that the two items Mrs and Ms are conventionally treated as abbreviations, even though they can be written in no other way.)
Guardian and Observer Style Guide
Mr, Ms, Mrs, Miss
The reader who questions the period after Ms. also objects to the repeated use of honorifics throughout an article:
Unless there’s a possibility of confusion, I see the repeated use of Mr., Ms, and Mrs. as an unnecessary courtesy–and annoying or actually silly at times.
Both American and British newspaper guides agree with our reader that the repetition of honorifics throughout an article is unnecessary.
The Guardian/Observer guide discourages the repetition of honorifics after their first mention, but allows some variation according to context:
In news stories particularly we should use an honorific if it sounds jarring or insensitive not to do so – for example, a woman whose son has been killed on active duty in Iraq should be “Mrs Smith” and not “Smith”. We need to use our judgment and be guided by the tone of the piece.
The AP rejects courtesy titles altogether, unless they occur in a direct quotation:
Refer to both men and women by first and last name, without courtesy titles, on first reference: Susan Smith or Robert Smith. Refer to both men and women by last name, without courtesy titles, in subsequent references. Use the courtesy titles Mr., Miss, Ms. or Mrs. only in direct quotations or after first reference when a woman specifically requests it; for example, where a woman prefers to be known as Mrs. Smith or Ms. Smith.
It’s futile to overthink arbitrary punctuation usage. Pick a relevant style guide and follow its recommendations.