chancellery vs head of government

chancellery

noun
  • The office or staff of a chancellor 

  • The rank or post of a chancellor 

  • The building in which such an office is situated 

head of government

noun
  • The chief officer of the executive branch of a government, often presiding over a cabinet; usually called Prime Minister (in a parliamentary system) or President (in a presidential system). 

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