Adding -er or -or to a verb generally makes a noun with the meaning of someone or something that performs that action. Is there any way to know which one to use?
I’m using a computer, so that makes me a computor. However, I can also be a processor, and the computer uses a processor. The job of one machine I worked with was to modulate, so it was a modulator; another’s job was to scramble, so it was a scrambler. What about a machine used to encrypt? Is it an encryptor or an encrypter? Would a person doing the job be different? My head spinning. (It’s a spinner?)
That’s a good question. I don’t know the answer, and I can’t see any logical rule jumping out at me, but I have noticed that here in the U.S. we spell the word “impostor”, while Elvis Costello (who’s from the UK; I’m just assuming this holds for UK spelling in general) spells it “imposter”. Which I always found odd for some reason. Sorry for the hijack. Maybe it holds a clue.
I’m not sure what the rule is. Whatever it is, it’s probably reasonably flexible to account for the different international usages: US English seems to have more “-er” words and fewer “-or” words than Commonwealth English (adapter/adaptor).
I don’t believe there is any rule. People pick one or the other until a common usage is determined.
My rule of thumb is this:
If the word is of Greek or Latin origin, use -or as the suffix.
If the word is of Germanic origin, use -er.
It’s not foolproof by any means, but it works for me.