There are 2 sizes
Standard tree: Fits most horses with a distance of 9" or less between the center of each longissimus dorsi muscle.
Shire tree: Fits most horses with a distance greater than 9" between the center of each longissimus dorsi muscle
Why does this matter?
A metal stirrup bar is attached to the layers of rawhide that make up the tree. If the stirrup bar is too narrow for the horses back, it can cause pinching & will be impossible to adjust the saddle to a level and balanced position as the saddle will be uphill no matter how you have the panels adjusted.
Please contact us if you have further questions or would like help sizing your horses back.