The more weight  carried on the saddle, the stronger it has to be, and a stronger saddle is a heavier saddle. The question is often asked:

“Why is not a military saddle like a plain saddle and as light as one?” 

The answer is that such a pattern could not carry the impedimenta, nor could a lightly constructed saddle possibly stand the strain to which a military saddle is exposed. It is obvious that the more weight it has to carry and support, the stronger it must be, and that the first step towards a light saddle is a light load.

Materials of arches
The arches of a saddle are made of wood or metal, or a combination of these. Metal is used in all military saddles constructed within the last 25 years; prior to that two pieces of beech were given appropriate curves and secured together with an arch of steel known as a gullet plate.

Front arch
The front arch forms the pommel, the rear arch the cantle; both pommel and cantle may be high  or low; they are made high, especially the cantle, when bulky material has to be carried on the saddle.

Strain on arches
The strain on a front is greater than that on a rear arch, for the reason that, owing to the shape of the horses body, the saddle is really resting on a wedge and this with the weight of the rider is constantly exercising an outward strain on the arch.  This strain is greatly increased where the fit is defective and excessive weight carried.