A jelly roll is a bundle of precut 2.5" strips of fabric cut WOF. It normally is one or two strips from each fabric in a particular line. since the number of fabrics in a line can vary, the number of strips in the bundle can also vary. Usually it's between 20 & 42, so you need to be careful when you buy to understand how many strips are in the bundle. Most commercial patterns that you see are figuring on around 40, but the pattern should specify the number of strips, not just 'a jelly roll'