Fabric has 3 grains: straight, cross and bias. Straight grain is parallel to the selvage edges and has very little or no stretch. Cross grain runs from selvage to selvage and has some stretch. Bias is 45 degrees to either selvage and has lots of stretch. If you sew squares together and match straight grain to straigt grain as well as cross grain to cross grain it's much easier to keep the seam lines straight and avoid puckers in the seams. If you sew bias to straight grain you can almost guarantee that the bias will be too long and the pieces won't fit when you sew them together.