I use perle cotton crochet thread and tie a granny knot (square knot) then a surgeons knot. That's how I was taught to do it and I have quilts that are 40 years old and none have ever come untied. I think all the baby quilts have worn out except one that Mom used as a pad for a rarely used play pen. I know others just do a double granny knot but my granny didn't always use cotton thread and thought the slick acrylic threads could & would untie if the quilts were washed often so she taught us to do it this way instead.