Pythagorean Theorem: Living on the Level and by the Square.
“The 47th proposition of Euclid’s first book of the “Elements”, also known as “The Pythagorean Theorem”, stands as one of Masonry’s premier symbols, though it is little discussed and less understood today. That fact is made the more unfortunate, since the 47th proposition may well be the principal symbol and truth upon which Freemasonry is based.”
Hermetic Rambling 