In fitting saddles, including treeless saddles, it helps to know if the saddle, and your horse, are more suited to "arch" or "apex".

Arch refers to a shape that is more similar to an upside-down "U" (mutton-withered).

Apex refers to a shape that is more similar to an upside-down "V" (higher-withered).

See the other pages about saddle fit and back shapes:

Back Shapes

Back Shapes 2

Saddle Fit Templates

Saddle Fitting Guides

Considerations for the Heavier Rider

Weight-Carrying Ability

Saddle Fitting

Saddle Slippage

Gullet Widths of Saddles

Icelandic Saddle Fit