Philip B. Corbett/The New York Times

Many topics come up repeatedly in reader comments and e-mail messages to After Deadline. Unfortunately I’m not able to offer a direct response to each comment (truth be told, After Deadline is a sideline for me). But one thoughtful reader suggested that I compile answers for some of the most common questions.

Here’s a start in that effort. I’ll add other topics as they come up, and I’ll link to this item from each week’s column so readers can find it easily.

[UPDATED on Nov. 14, 2011; newest item on top.]

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Why Do Plural Abbreviations Have an Apostrophe?

A number of readers have complained after seeing plural abbreviations likeM.D.’s and M.R.I.’s in The Times. They say the apostrophe is wrong — that it indicates a possessive and should not be used in a plural like this.

In fact, style rules at The Times (and some other publications, including the Chicago Manual of Style) do call for using an apostrophe in the plural of abbreviations that include periods. The idea is that a combination of uppercase letters, periods and a lowercase “s” is confusing at first glance, and that the apostrophe helps a reader see that the “s” has been added to make a plural.

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