either, neither

The quantifiers either and neither are a kind of determiner. We use them to talk about possible choices between two people or things.

When used as a determiner, either and neither must come before a singular countable noun.

either

Either is positive (+). It means "one OR the other" (of two).

• A. Do you want to meet on Monday or Tuesday?
  B. Either day is good for me. (Monday is good for me. Tuesday is good for me.)
• A. There are two keys here. Which will open the office?
  B. I believe either key works.

neither

Neither is negative (-). It means "not one AND not the other" (of two).

• Neither team scored and the game was a draw. (Team A didn't score. Team B didn't score.)
• Neither player was allowed to return to the game after the argument.