when you attach your strips with corner stones be sure your seams are pressed in opposite directions. so if you press the corner stone seam toward the strip then the piece you are adding it to should be going in the opposite direction then when you put them together you can snug those seams together for a perfect match...if the edges are off a tad they can be trimmed but if your join is whats off this will help. some people even make them a little bit larger so they can trim after attaching. but nesting your seams makes a huge difference when joining...corner stones or not.