149 The Period

Though a seemingly trivial punctuation mark, the period does present some knotty challenges, especially in technical writing. We all know to place a period to signal the termination of a simple sentence that makes a statement. However, here are a few more specialized rules:

• Do not use a period in combination with other punctuation marks unnecessarily, especially when a quotation is involved. In such an instance, end the sentence naturally on whatever punctuation mark is logical (e.g., a question mark).
• Avoid using periods at the ends of abbreviated units of measure, except when the period might be confused with another word. (Therefore, so that it’s not confused with the word “in,” use “in.” to abbreviate “inches.”)
• When using a period in conjunction with parentheses, the period comes after the parentheses are closed if the parenthetical comment itself is part of the larger sentence (as in the first bulleted sentence above, and this one). The period comes inside the parentheses only when the parentheses themselves contain a complete independent sentence. (See the example in the second bulleted sentence above, as well as this sentence.)
• By convention, if an abbreviated word (such as “etc.”) ends a sentence, let a single period signal the sentence’s end—two periods in a row would be incorrect.
• In acronyms commonly understood or commonly used in your field (ASTM, EPA, US, GIS), do not use periods after the capital letters.
• Do use periods after abbreviations and acronyms that are forms of address, initials within proper names, earned degrees, and when expressing measures of time (Dr. Bauer; M.S. degree; Steven S. Wilson, Jr.; 5:00 p.m.; 10 B.C.).