Q. Does CMOS prefer a slash or parentheses to denote an alternative? For example, “on/off” vs. “on (off).”
A. Parentheses and slashes can both denote alternatives, but the use of parentheses in that role is limited. For most alternatives, the slash is best. The form “on/off” means either “on” or “off.”
The form “on (off),” on the other hand, would tend to suggest (illogically) that “on” is equivalent to (rather than an alternative for) “off.” To fix that, you’d need to add an “or”: “on (or off).”
But parentheses can be useful for alternative word endings. For example, instead of “return your manuscript to the author or authors” or “author/authors,” a more concise form is “return your manuscript to the author(s).”
That works best with simple s or es plural endings, in which the parentheses show a letter or letters that would be added to the term. Anything more than that—e.g., “warranty(ies),” in which “ies” is an alternative to “y”—though useful in a pinch, can quickly start to become unclear.