Q. In CMOS 10.3, I am confused by the meaning of the following sentence, describing “less familiar abbreviations”: “Such an abbreviation should not be offered only once, never to be used again, except as an alternative form that may be better known to some readers.” Would you please clarify? Thank you!

A. Normally, the point of introducing an abbreviation is to save space on subsequent mentions.

The National Endowment for the Arts (NEA) was created in 1965 through an act of Congress. NEA grants have gone to museums, educators, . . .

But it can be helpful in certain cases to give both forms simply as an aid to readers who might know the abbreviation better than the spelled-out form. For example, the International Organization for Standardization is known to many people as ISO, so it would be helpful to include that form even if you were to mention that organization only once. For example,

Times have been presented according to the latest recommendations from the International Organization for Standardization (ISO).

There’s a chance that a copyeditor might query that (“AU: ISO doesn’t appear again. Delete here?”). The author, however, would be right to stet such a suggestion.