The stretched bars on my Economy canvas have slots in the corner, why?

The slots in the corners of the stretcher bars are used to tighten the canvas using little wooden wedges. The little wooden wedges are called canvas keys. Over time canvas can become loose or saggy on the stretcher bars. To tighten the canvas back up you put the keys in the slots made for them in the corner of the bar and tap them in further to square up the canvas.