How to use either.

Let’s start with the basics. 

1. either + or = one of two choices (NOT both)
(conjunction)

• Your order comes with either soup or salad.
• The movie starts at either six or seven. I don’t remember. 
• We’ll finish the project on either Thursday or Friday. 
• A: Do you want coffee or tea? 
  B: Either (coffee or tea) is fine. 

2. either = links with a similar, usually negative, idea
(adverb)

• Charlie’s not a bad singer, but he’s not great either.
• A: There’s no milk in the fridge.
  B: We don’t have any eggs either.
• A: I don’t like this movie at all. 
  B: I don’t either.  
(For more on negative agreement, see here.)

3. either + of = one or the other of two people or things
(determiner)

• Maggie, Ben? If either of you is free, I need a little help.
• Don’t tell either of my roommates, but I’m going to move out.
• One of your parents has to sign this paper. Either of them is fine.

TAKE CARE  
either = both
(determiner) (notice the plural “s” with both)

• There are farms on either side of the road. 
• There are farms on both sides of the road. 

• There is a goal on either end of the court. 
• There are goals on both ends of the court. 

This is a common expression, but it can be confusing to some readers. Some writing guides suggest avoiding it.