Don't know if it matters, but I've often used a knife that has been sharpened in a certain way to cut tight stitches. It's sharpend without removing all the tiny, tiny serarations made by the stone, and it almost saws through the stiche. So I stick the point between the stitches, pull the seam apart a little, then move the knife just a little. It saws the stich on a very small scale, if that makes sense.