About 28 families worldwide.
10 families in North America.
18 to 22 body rings. 20 body rings is the most common. Dorsal groove absent. Large paranota (wing-like lateral extensions of the body segment) give a flattened appearance to many species, although paranota lacking in some species. Members of some families vividly colored with red, yellow, orange and black.
The largest millipede order, and the only one that produces cyanide as a defense.(1)
Some Chordeumatida (e.g. in Rhiscosomididae, Striariidae, Tingupidae) have prominent paranota and a similar "flat-back" appearance, but possess >25 body segments.