Paul J. van Doren, Gordon C. Lee, and Robert E. Diamond received US Patent 4,783,909 in 1988 for a "Reversible Heel Counter for Shoes." The zigzag stitches at 44 and 46 allowed the back of the shoe to bend and act like a hinge, "so the shoe can be selectively used as either a slipper or a regular shoe." Courtesy of USPTO