Q. I am editing a brief in which the author has used “and/or” multiple times. I know that this term should be avoided, but I’m not exactly sure why. Is it because it’s confusing and ambiguous? What is The Chicago Manual of Style’s stance?
A. CMOS, in chapter 5, says to “avoid this Janus-faced term” (5.250, s.v. “and/or”). Janus-faced means duplicitous—in other words, appearing to say two contradictory things simultaneously. The problem is the slash, which is potentially ambiguous; for example, readers might choose to interpret “x and/or y” as meaning either x and y or just y—but not x alone. In fact, “x and/or y” is usually intended to mean “x or y, or both,” and where that is the case, section 5.250 recommends writing exactly that (take a sleeping pill or a warm drink, or both). In many cases, however, “or” alone would make the meaning perfectly clear. For example, “no cats or dogs allowed” means that no combination of cats or dogs—or cats and dogs—is allowed. In formal prose, including legal writing, such considerations of the precisely intended meaning are important. In casual prose, “and/or” can occasionally serve as a useful shorthand: bring your own beer and/or wine. No one will fail to understand the meaning of that.