to me a crumb is any piece of fabric that is too small for using as a part of a block pattern. I have made 100's of crumb blocks and several crumb quilts- one of my favorite blocks to make because there isn't a "wrong" and it's mindless sewing- which sometimes is what I need, just to be one with my machine and beautiful little pieces of colorful fabrics. I have a tote full of just crumbs, and a tote of just strings. If the crumbs are really too tiny then they go into pet beds.