Have you ever wondered why quilt shops sell long strips of fabric in 2 1/2 inch widths? Or how about the package of 4 1/2 inch quilt squares sold at quilt shop?
4 1/2 inch sized quilt squares are referred to as quilter's squares. This is the size of quilt squares that most experienced quilters purchase. Here is why. When you sew quilt squares together or into projects, you lose 1/4 inch from all the outside edges in the seam. Hence when a 4 1/2 inch quilt square is put into a project, it becomes 4 inches. Some people refer to 4 1/2 inch quilt squares as having the seam allowance built in.
When you sew together a couple 2 1/2 inch wide strips of fabric, you end up with a strip that is 4 1/2 inches wide. This strip can then be cut into 4 1/4 inch squares.
This makes measurements, and calculating the size of patches and quilts easier. Unfortunately selling 4 inch quilt squares became popular on ebay in the 1990's and continues to this day. You can do a lot of things with these squares, but they don't have the versatility and ease of 4 1/2 inch squares.
Quilter's measurements also can make the size of quilt blocks similar. Quilt blocks are usually made with grids. See Patches and Grids. The most common grid is the 9-patch. Depending on whether you use 2 1/2, 3 1/2, or 4 1/2 inch grids (or squares), your block will be 6x6, 9x9 or 12x12 inches in size. It is easier to figure the math when you can just discount the 1/2 inch because it is going in the seam allowance.
Grid are further divided in more complex quilt blocks Again, it is easier to do the math when you start with a measurement that already has the seam allowance in it. For instance, if you are breaking up a 4-inch grid into 4 strips of different fabrics, you would cut all the strips 1 1/2 inches. When sewn together, they measure 4 1/2 inches.
Many quilters like to experiment with making new blocks and then they set them aside. When you make your blocks with quilter's measurements, they are easy to work into a project with other similar sized blocks at a later time.
If you like to organize leftover bits of fabric, you can cut some of it into strips of 1 1/2, or 2 1/2 inches. These are great to have ready to go when you just want to sit at the sewing machine and sew making scrappy quilts.
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