It used to be that fabric came on bolts and you bought yardage depending on the amounts indicated on your dress pattern.
Cotton fabric like the muslin shown here usually comes in a width of 44 inches (folded in half) rolled in a bolt several yards long.
With more and more people taking up quilting and crafting, there has been a need for another way to buy fabric: the fat quarter. This quilt for an American Girl doll bed does not take much fabric.
Fat quarters are pre-cut pieces of fabric that are 1/4 of a yard. They are usually folded like this at the quilt shop for easy display. They are also pre-cut into bundles of coordinating fabrics when buying online. The difference is that it is not cut in the traditional way that fabric has always been cut.
One yard of muslin looks like this. It is 44 inches wide (when unfolded) by 36 inches long.
A traditional cut of 1/4 yard would look like this compared to a full yard of fabric.
It measures 44 inches wide and is cut at 9 inches long (1/4 of 36 inches).
A fat quarter looks like this compared to a full yard of fabric.
It still is a quarter of a yard but it measures only 22 inches wide (half the width) and is cut at 18 inches long (half the length of a yard).
Both pieces measure 1/4 of a yard. The traditional cut is skinny compared to the new cut which is "fat". Depending on the project, the fat quarter can provide a more usable piece of fabric.