A number of years back, my friend Pete Gorrell (a veteran saddlemaker now living in Colorado) showed me how he shims his stirrup treads. I add one thick layer of skirting leather to the inside tread of the stirrup, which is the side nearest the horse. As the stirrup leathers drape around the barrel of the horse, the inside of the tread will be lower than the outside. One might think that your leg would drape the same way, but the difference is that our leg articulates at the joints. This creates a situation where there is nearly a straight line from the knee to the sole of the foot with the bottom of the foot fairly plumb with the ground. The idea of the shim is to create an even distribution of pressure in the tread. I’ve ridden with shims in my stirrups for quite a few years and find that my feet and joints are more balanced and comfortable. My customers report similar feedback. The theory holds that if a person is pronate, (that is, slightly bow-legged) this will be helpful. If the rider is supinate, (that is, knock-kneed) this could make the rider more uncomfortable than not doing anything at all with the treads. This is something that is changeable if ever the need arises. Riders can experiment with swapping the stirrups around so that the shim is on the outside of the tread for the sake of comparison.