both vs either

both

det
  • Each of the two; one and the other; referring to two individuals or items. 

pron
  • Each of the two, or of the two kinds. 

conj
  • Including both of (used with and). 

either

det
  • Each of two; both. 

  • Any one (of two). 

conj
  • Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”. 

pron
  • One or the other of two people or things. 

adv
  • As well. 

How often have the words both and either occurred in a corpus of books? (source: Google Ngram Viewer )