Two-way lock nuts are also called reversible, center-lock, or side-lock nuts. There is a small indentation in the middle of one or more sides on the outside of the nut; it looks like a punch right in the center. The number of punches (two is the most common) shows it’s a two-way nut, as the name suggests.

Both Stover(R) type lock nut and two-way lock nuts use distorted threads to keep the nuts from loosening. The difference is that the distorted thread of Stover nuts is at the top and two-way nuts have it located in the center where the punch is. These also come one and three side punched though those much less common.