The term “thread per inch”, or “TPI,” refers to the size of the attachment surface on the muzzle of a barrel. Technically TPI measures the number of spiral threads in one inch. Many rifle barrels—and some handgun barrels—have screw-on attachment points at the muzzle to allow for muzzle brakes, flash hiders, and suppressors. However, not all barrels are the same size or chambered for the same caliber, so it is important to note the thread per inch of a specific barrel to understand which accessories can be attached.