If the surface be uneven, the cutter will not cut at all, or but little, in passing over low places, since the toe and heel of the sole will then be resting on higher places; but when the cutter reaches a high place a shaving will be taken off.

Hence it follows that the longer the plane, the straighter will be the surface produced.

The length of the plane used is determined by the length of the wood to be planed, and the degree of straightness desired.
The part of the sole directly in front of the cutter presses firmly down on the wood and so prevents the shaving from splitting far in advance of the edge.

It follows that the narrowness of the mouth in a plane is an important factor in the production of smooth surfaces.
This can be regulated by adjusting the toe in the block-plane, and by moving the frog in the jack- and smooth-planes.
A recent improvement in jack-, smooth-, and fore-planes consists of an adjustable frog, by means of which the throat can be narrowed or widened at will by means of a set-screw in the rear of the frog without removing the clamp and cutter.

It is made by Sargent and Company.