In right-angled triangles the square from the side subtending the right angle is equal to the squares from the sides containing the right angle.
Euclid was a Greek philosopher who is called the father of Geometry. He lived in Egypt around 300 BC. He wrote one of the most influential mathematical textbooks in all of history: the “Stoicheion” or Elements. In it he captured much of the mathematical achievements of ancient Greece.
It is also referred to as the 3:4:5 ratio, or the Pythagorean Theorem. It is related to the concept of sacred numbers. This mathematical principle is used to create perfect squares, and was highly important in the laying of foundations during the building of temples and palaces in the ancient times, including ancient Egypt, Babylon, Greece and Rome.
It is also used to remind Freemasons to “square their square when it gets out of square”.
More Info:
https://www.masonic-lodge-of-education.com/47th-problem-of-euclid.html
http://www.freemasons-freemasonry.com/euclid_unveiled.html