Almost all stretched artist canvas comes with a small bag of flat wooden or plastic sticks. These sticks are called "canvas keys" and can usually be found stapled to the pack of the stretcher bars. The canvas keys are a couple of inches long, with a flat end and a pointed end.
Canvas keys are only included in midprofessional to student-grade stretched artist canvas. High-end artist canvas tends to include stretcher bars with corner brackets, which you can tighten or loosen to ensure the canvas remains square and the fabric taut. Most of the canvas that may need canvas keys is sold pregessoed and ready to use. In some student-grade canvas panels, the canvas keys are actually made of molded plastic.
It is very important to have a taut painting surface; the canvas keys help to strengthen and tighten the canvas. Canvas keys also keep the canvas square, and help it to lay flat against a wall when displayed. Canvas keys are not always necessary. If a canvas has a sagging painting surface or is slightly warped, then it may need canvas keys. In some cases, canvas keys cannot adjust the canvas enough to solve the problem.
To use canvas keys, slip the pointed end of the key into the slot on the stretcher bars, and gently tap into place with a hammer. Add another into the adjacent slot, and tap it into place. The two keys pressing against one another will force the canvas frame to remain square and tight. Typically, you'd insert canvas keys after the painting is completed, but you can insert them at any point during the creative process.
If your canvas is loose but it did not come equipped with canvas keys, then there is a way to ensure the canvas will remain taut. Because canvas is a tightly weaved material and shrinks as it dries, you can simply spray the back of the canvas with lukewarm water from a misting spray bottle. As it dries, the canvas will shrink slightly, rendering a tighter surface. Repairing a canvas that is out of square is a little more difficult without canvas keys. You can create canvas keys out of ice pop sticks or scrap wood, or you can screw brackets straight into the stretcher bars to reinforce the corners and ensure the artwork remains in square.