alliance vs civil union

alliance

noun
  • A union or connection of interests between families, states, parties, etc., especially between families by marriage and states by compact, treaty, or league. 

  • Marriage. 

  • The persons or parties allied. 

  • The state of being allied. 

  • The act of allying or uniting. 

  • Any union resembling that of families or states; union by relationship in qualities; affinity. 

civil union

noun
  • A legal union similar to marriage, established to allow same-sex couples (and, in some jurisdictions, opposite-sex couples) rights similar to those married couples have. 

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