explain vs spell out

explain

verb
  • To make plain, manifest, or intelligible; to clear of obscurity; to illustrate the meaning of. 

  • To give a valid excuse for past behavior. 

  • To make something plain or intelligible. 

spell out

verb
  • To explain in clear and simple terms; to be explicit. 

  • To form (a word) from letters; say the letters in a word. 

  • To replace an acronym with its expansion. 

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