When to Stop Defining Abbreviations

Back when I taught copyediting, my students once asked me when it would be OK to not spell out an abbreviation that doesn’t appear in the dictionary. I appreciated their confusion, because it is sometimes difficult to define an exception to the rule.

We used The Chicago Manual of Style in the course I was teaching, and even CMoS isn’t entirely clear on what the exceptions would be (17th ed., 10.3)

A number of expressions are almost always abbreviated, even in regular prose, and may be use...