The tree of a saddle is the firm, inner part that gives the saddle its stability and main shape. This frame is usually made of plastic or wood and comes in different sizes to accommodate the differing shapes of horses' backs. English saddles have varying tree widths, so what fits one horse might not fit another.
Place your saddle on a rack that will allow you to easily view the front of the saddle, maneuver the saddle flaps and use a measuring tape.
Locate the tree points on your saddle. The tree points are considered the lowest parts of the tree and there is one on each side. A tree point is the portion of the frame that comes down the farthest on the horse's back and will sit slightly below and behind the horse's withers. Lift up the saddle flap and look in front of the billets—the straps that you connect the girth to—in order to locate the points. You will be able to feel them.
Measure across the front of the saddle from one point to the other. Use the bottom of the tree points as your measuring locations. Do not bend the measuring tape around the saddle to touch the point; instead, imagine a line that runs from the bottom of the tree point to the front of the saddle and measure from there. You need a straight line across the front of the saddle.
Take your measurement in centimeters. It is now time to determine the width of the tree. If your measurement was approximately 28 cm, the tree is narrow. If you got 29 cm, it is standard; 30 cm is medium. A tree that is 31 cm is wide and one that is 32 cm is considered extra-wide.