On the loom the long threads run from the top of the frame to the bottom, side by side, and they never meet. Keeping them parallel is the first job of the morning and the one the weaver checks most often, because everything after it depends on it.
The pattern comes from a thread that crosses them at a slant. Where it passes the first long thread it makes a certain angle; where it passes the second it makes exactly the same one, and it goes on making that same angle at every thread it crosses, all the way across the cloth.
That repetition is the pattern. A diagonal stripe on a rug is not drawn stripe by stripe; it is one crossing angle, repeated because the threads beneath it are parallel.
The weaver's son once pulled the frame slightly out of square while cleaning it, and the long threads were no longer quite parallel. Nothing looked wrong at the top of the cloth. Half a metre down, the stripe had begun to bend, because the angle it made was no longer the same at every thread.
They cut the piece out and started again, and the weaver used the loss to make the lesson stick. Parallel lines give you one angle repeated for as long as you like. Lines that are almost parallel give you an angle that changes so slowly that you notice it only when the work is finished.