Measuring against the linchpin
Measuring against the linchpin was a method of determining which enemy civilians would be beheaded, used by the Mongols and other tribes that lived in the Mongolian plateau. Genghis Khan used this method against Jamukha's coalition of tribes in 1202. All male captives were forced to walk beside a wagon wheel. If their heads were higher than the linchpin (a pin inserted at the end of the axle) they were immediately executed. The wagon wheel was a large wheel used to transport yurts and other goods. This technique was probably used to preempt against revenge  attacks that were common between tribes at this time. If one tribe were to attack another, there was always the possibility that there would be a revenge attack soon after. By eliminating the older males, there was less chance of a counterattack from tribes that were in perpetual conflict due to centuries of distrust and robbery.