In looking over past A-to-Z’s I notice a lot of my U- entries are the negation of something. Unknots, for example. Or unbounded. English makes this construction hard to avoid. Any interesting property is also interesting when it’s absent. But there are also mathematical terms that start with a U on their own terms. The Summer 2017 Mathematics A To Z: Ulam’s Spiral shows off one of them. Stanislaw Ulam’s spiral is one of those things we find as a curious graphical adjunct to prime numbers. The essay also features one of my many pieces in praise of boredom.