text: when measuring a triangle what is meant by 3 4 5 rule?   the 3 4 5 rule is an easy way to mark a right angle in the field.  the ancients, even before pythagorus, discovered that if you took a length of rope and knotted it so that one side was three of whatever units, the second side was four of whatever units, and the third side was five of whatever units, then you would get a right angle.  devices using the 3 4 5 rule were used in ancient construction -- including the pyramids of egypt.