Punctuation / A Primer of Information about the Marks of Punctuation and their Use Both Grammatically and Typographically

by Frederick W. Hamilton

The metric symbols are treated as abbreviations but the chemical symbols are not. M. (metre) and mg. (milligram) but H₂ O and Na Cl.

Per cent is not regarded as an abbreviation.

The names of book sizes (12mo 16mo) are not regarded as abbreviations.

The period is now generally omitted in display matter after

Running heads,

Cut-in side-notes,

Central head-lines,

Box heads in tables,

Signatures at the end of letters.

The period is omitted