Hand someone a new pen to try and there is a good chance they write their name. Not a word, not the date: the name, or else a tight little spiral. It makes sense once you see it as a test. To find out whether a pen is any good you need to write something whose shape you already know exactly, so that any skip or blot or wobble can only be the pen’s fault. You have probably written your name more often than anything else. You know how it goes down to the angle of the last stroke. So the name is a ruler, and you lay the pen against it to see where the pen comes up short. Signatures drift over a lifetime, of course. But for the length of one scribble on the back of a receipt, yours is the fixed point, and the rest of the world is the thing being checked.