Quilting and "Cutting Down."
In order to achieve a perfectly sized quilt block, many quilters will oversize a quilt block, and then cut it down to the perfect size.
It can be hard to make quilt blocks the perfect size. First of all, you have to cut your fabric accurately. Then you must sew the perfect 1/4 inch seam. You also have to remember to sew a "scant" quarter inch seam. You have to make the seam a thread shorter than 1/4 inch. This is because when the block is pressed and the seam turned, the thickness of the thread will affect the size of the block.
Fabric is fluid. If your blocks are large, you can often stretch two blocks that aren't quite the same to fit. However, this results in one side basically being "gathered", and it will result in unsightly puckers, even if they are small.
The goal in piecing is to make your work as flat as possible. When you oversize a piece, and cut it down, your block will be perfect. You don't waste much fabric, but it is an extra step, and not all quilters do it.
It you are cutting down a block like a half square triangle, you can use a square ruler and make just one cut. However, if you are cutting down a block like a 4-patch, which has a line down the middle, you must remember to divide the amount you are going to cut down by two and cut off the same amount on each side of the center line.
The goal in piecing is to become so good at cutting fabric, and sewing with the 1/4 inch seam that cutting down is not necessary. Until then, "cutting down" is a way to make a very attractive quilt that goes together perfectly.
Here is a picture that illustrates cutting down a half square triangle. You would not oversize a block this much, but I wanted to clearly illustrate the lines. Notice how the diagonal line is matched to the fabric, was well as the 3 inch horizontal and vertical lines. All you need to do is make one vertical cut on the ruler and one horizontal cut.
If, however, the bottom of your block is not straight, you will need to cut twice. In this illustration, I am cutting down to a 3-inch block. I would first cut it down to 3 1/4 inches, and then turn it around and cut the other side so the block will be straight on the bottom also.
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