Thermal Stitch
a dense, double-thick, reversible fabric made by working into rows below to trap an insulating layer
Thermal stitch makes a double-thick, insulating fabric by working each stitch into a loop from two rows down, which traps a hidden layer of yarn between the faces. You crochet a single crochet not into the row directly below but into the free front loop of the row before that, so the untouched loops of the intervening row get caught inside and the fabric quietly builds two joined layers. The result is thick, sturdy, and identical on both sides, with real heat resistance, which is exactly why it is the classic choice for potholders, hot pads, and trivets, as well as warm bags and cold-weather accessories. It works up slower and heavier than ordinary single crochet because there is effectively twice the fabric, and it uses noticeably more yarn. The one thing to learn is spotting the correct loop two rows down; going a row too shallow or deep changes the thickness. Keep the tension even so the trapped layer sits smoothly and the fabric stays flat.