There are certain classes of back which especially lend themselves to injury by both burr and fan.  For instance, a hollow back is certain to be injured by both ends of the side by and the reason of this is evident.

A horse with thick low withers, with blade bones comparatively wide apart, is peculiarly liable to injury from the burr.

When muscle waste becomes established, the edge of the blade bone at the play of shoulder becomes more and more evident, as the long muscles of the back melt away, until it stands up like a new growth.

Under these conditions, the burr will press on it and do injury, as the muscles of the back having wasted, there is nothing to keep the saddle off the blade bone.

Distance between side bars.
It is almost impossible to have the channel between the bars too wide.

One great reason for this is that circumstances necessitate a blanket should be carried under the saddle; and if the distance between side bars is insufficient, the thickness of these blankets will cause the side bar to rest on the side of the withers, for it is obvious that the tendency of a saddle blanket and numnah is to fill up the interval between the side bars.