pro forma vs rigid

pro forma

adj
  • Occurring, undertaken, or performed as a perfunctory matter, in the interest of form, or for the sake of formality. 

  • Of or relating to a document of form, especially an invoice sent in advance. 

  • Showing projected costs, liabilities, etc., especially as a result of an expected action or situation. 

adv
  • Undertaken or performed as a perfunctory matter, in the interest of form, or for the sake of politeness. 

noun
  • A document that shows the standard entries of a form or similar document. 

rigid

adj
  • Rigorous and unbending. 

  • Stiff, rather than flexible. 

  • Fixed, rather than moving. 

  • Uncompromising. 

noun
  • An airship whose shape is maintained solely by an internal and/or external rigid structural framework, without using internal gas pressure to stiffen the vehicle (the lifting gas is at atmospheric pressure); typically also equipped with multiple redundant gasbags, unlike other types of airship. 

  • A bicycle with no suspension system. 

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