When clipping corners, reinforce the corner with a double row of stitching then clip the corner at a slight angle close to the stitching. The double row of stitching will make the corner stronger and help prevent accidents when turning it, and the angled cut will allow the seams to sit flat so that the corner forms a perfect angle. A point turner is an inexpensive and handy tool for turning points. You can find point turners in your local sewing center, or here online at Amazon.com.
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