The stretcher is necessary

A stretcher consists of four flat bars of wood mortised together at the corners somewhat after the fashion of a boy's slate, the pieces being cut so as to fit into each other closely, yet without glueing. These bars may be any width or thickness desired, according to the size of the canvas, from an inch and a quarter wide upward, and from half an inch to an inch in thickness.

The four corners being joined, the sides form a square or oblong square, which is kept in shape permanently by the canvas. The upper side of the bars, upon which the canvas is laid, should be perfectly smooth, the wood being planed off evenly, and beveled slightly inwards. To complete the stretcher a set of light, flat, thin, wedge-shaped, triangular bits of wood are made, two for each corner. The pointed ends of these wedges, which are called "keys," are inserted between the joints at the angles of the stretcher; the object of this is to tighten the canvas by separating the corners of the stretcher as much as necessary. The keys are of course on the under side, and a slight tap from a hammer on each key in succession will cause the mor- tised joints to spring apart as far as the canvas will permit. This operation is called "keying up" a canvas, and is very necessary, when it becomes wrinkled or hangs loosely on the stretcher.