The old man who made kites for the children in the square worked to one rule, and he said it in the same words every time: the two sides must be the same.
He measured them with a piece of string rather than a ruler, folding it in half to check. Then he set the cross-piece so that the frame folded onto itself exactly. If it did, the two angles at the bottom matched, and he did not need to measure them at all.
A boy asked whether it really mattered, since the difference would be a millimetre or two. The old man let him make one that was slightly off, and they took it out together.
It flew, and it pulled to the left. The wind pushed a little harder on the longer side, and the kite turned into that push, and kept turning, and had to be brought down.
The old man's point was not about kites. Equal sides give equal angles, and the wind meets both sides at the same slope, so it pushes equally and the kite goes where it is pointed. Unequal sides give unequal angles, and the wind notices a difference far too small for a boy's eye to see.
"You cannot measure an angle in the sky," he said, giving the boy the string back. "You can measure two sides on the table, and the angles look after themselves."