How did the egyptians used the knotted rope to measure right triangles?

It is believed that they used a piece of rope, in the form of a loop, with 12 knots that were equidistant. If knots 1, 4 and 8 were attached to pegs and stretched out, they made a triangle of sides 3, 4 and 5. Then since 32 + 42 = 52, the pegs formed a right angle. Increasing the length of the rope and so the distance between the knots improved the accuracy.

Note that the longest side goes from knot 8 to knot 13 which is actually knot 1.