In Information Technology, we often see something that looks like a compound adjective, pairing a number and a noum. Some examples are two-factor authentication, two-step verification and twelve-factor app. I always found it amusing that the noun itself is not plural.
Why isn't it named, for example, "two-factors authentication?" Is there a rule for that construct? If so, what's its name?
(My theory is that it probably came from multi-factor authentication, for example, in which there is some sense in the format, but I don't know for sure.)
