GEOMETRY

Set to 3 different clues across 3 published puzzles, most often “Euclid's subject” — The New York Times, Wednesday, 8 September 2010.

8 letters

Every clue on record, most used first

Euclid's subjectonce
The New York Times — Wednesday, 8 September 2010
Class of planes?once
The New York Times — Sunday, 12 February 2006
"There is no royal road to _____": Euclidonce
The New York Times — Monday, 17 October 1994