rondel vs roundel

rondel

noun
  • A rondelle, (small) circular object. 

  • A small round tower erected at the foot of a bastion. 

  • A long thin medieval dagger with a circular guard and a circular pommel (hence the name). 

  • A metric form of verse using two rhymes, usually fourteen 8- to 10-syllable lines in three stanzas, with the first lines of the first stanza returning as refrain of the next two. 

  • The verse form rondeau. 

roundel

noun
  • A bastion of a circular form. 

  • A circular insignia painted on an aircraft to identify its nationality or service. 

  • A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries. 

  • Anything having a round form; a round figure; a circle. 

  • A circular spot; a charge in the form of a small coloured circle. 

  • A roundelay or rondelay. 

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