What is the difference between full-quarter horse and semi-quarter horse bars on western saddles?

The terms used in saddle tree sizing are explained in Richard Sherer’s book on western saddles, and they deal with the distance between the bars, measured at the gullet. These widths may not be consistent between different saddle tree makers, but it gives you a starting place. The width of your horse’s shoulder determines the type of tree and gullet width you will need.
Regular quarter horse bars – gullet width: 5 3/4″
Semi-quarter horse bars – gullet width: 6″…