The width varies from shop to shop, from about a centimeter and a half to over ten centimeters. The thickness does not vary much: roughly one millimeter. Lift a length of it out of the bowl and you can see through it.

That thinness also means it cooks faster than ordinary udon, which turns out to be the practical fact underneath everything else.

Nobody is certain where the name came from. One account traces it to imokawa udon, a flat noodle from Kariya in Aichi, worn down in pronunciation over time. Another says it describes a sash being rinsed in a river. Even the old established shops say the origin has not come down to them.

Kiryu was a weaving town. A great many women worked in the textile workshops, and what they wanted at midday was something that could be put in front of them immediately, which a noodle a millimeter thick can be. Plenty of customers bought a single portion and carried it home. It was originally simmered or served in broth, though some places now offer it cold to dip. One long-established shop in the town has been making it since 1887 and is now on its sixth generation, and even there the origin of the word has not survived. The only fixed rule about this noodle is that it is wide.