Are you “beside” yourself?

Beside or besides? That is the question. Both are prepositions (but “besides” can also function as an adverb: She traveled to three countries besides France.)

As prepositions, beside means “next to” and besides means “in addition to”.
The guest sat beside the host; no once besides the guest had that honor.

Now consider: There was no one beside her in the room. (Was she alone or was no one next to her?)

We get comfortable using beside as an adverb at times. Boo hoo! Make a note in Your Style Manual and don’t worry about it again.