Cornerstones on quilts refer to squares found on each corner of a quilt where the quilt features a border. The cornerstones are of a different, usually contrasting, color than the rest of the border and create a structural effect within the quilt's design. While cornerstones lend a professional, complicated look, they are simple to do and the technique can be applied to any type of quilt you want to make.
Cut two strips of the fabric you want for your border. The strips should be as long as the top and bottom of the quilt and as wide as you want the border to be.
Make two more strips from the border fabric. These strips should be as long as the vertical sides of the quilt and the same width as the other strips.
Measure the width of the border strips and cut four squares that width from the cornerstone fabric. So if your border is 3 inches wide, cut four squares that are 3 inches by 3 inches.
Sew the top and bottom strips along the top and bottom length of the quilt with the sewing machine. The top and bottom strips should be even with the top and bottom of the quilt. Press the seams open so the borders lay straight.
Sew one square to each end of the vertical strips.
Attach the vertical strips to the vertical sides of the quilt as you did for the top and bottom. Make sure the seams of the cornerstones line up with the seams of the borders of the top and bottom strips when you sew the sides in place.
You can make a multicolored border by using the same technique on the same quilt.
Do not rush through the cutting of your pieces or they will not come out straight and will mess up your quilt's symmetry.