Always spell out terms on first use, and introduce the acronym. Right? Well, almost. When the acronym is more familiar than the full name, it’s preferable to use the acronym rather than spelling it out in full. It might not even be desirable to spell out the acronym at all:
Some acronyms have even lost their claim to capital letters, they have become so familiar:
Acronyms Used Only Once
Sometimes, an acronym is introduced only because it is the more familiar version of the term. For example, a text might write out portable document format then mention the acronym PDF only because the acronym is more familiar, not because the acronym will ever be used again. And that’s ok too.
If It Is Spelled Out, How Often?
“In a work with few abbreviations and no list [of abbreviations],” says the Chicago Manual of Style (17th edition), “when an abbreviation reappears after a long interval in which it is not used, it may be helpful to repeat the spelled-out name as a reminder.”
In books that might be dipped into and out of, rather than read from one chapter to the next, it can be advisable to repeat a full, spelled-out term with its abbreviation in each chapter.
Like everything else in writing and editing, the rules for acronym use are context dependent.
Photo by Martin Postma used under BY ND-2.0 license.