From Book Description: Garter stitch squares are knit in garter stitch because it is the "squarest" stitch we have. The squares start out as a straight piece of knitting, but a decrease up the center "miters" the stitches into a square. If you used stockinette stitch for mitered squares, they wouldn’t be square; they would be more of a rectangle, or a diamond if placed on-point. Then you would have an interlocking diamond.