concordat vs confederacy

concordat

noun
  • A formal agreement between two parties, especially between a church and a state; specifically, an agreement between the Pope and a government. 

confederacy

noun
  • A state where the sovereign constituent units delegate their authority to the centre. As opposed to a federation, where the central and regional governments are each equal and sovereign in their own sphere. 

  • Specifically, an instance of a decentralized governing structure among the indigenous peoples of North America. 

  • An alliance. 

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