Proper track nuts have integral/captive washers. They permit easier adjustment of chain tension, and hold the wheel in place better than standard nuts with separate washers. Make sure to lubricate the threads and the crack between the nut and the washer.
10 x 1 mm size fits Harris/Formula/Iro, Shimano, Surly rear hubs and most freewheel hubs.
9.5 x 26 tpi (3/8" x 26 tpi) size fits Sovos and Suzue rear hubs shown (Except for Suzue NJS)
9 x 1 mm size fits Shimano front hubs and most freewheel hubs