Crochet is a pathway of chain loops that embed permanent metadata in every stitch. A hooked tool is necessary to get many of the effects. As crochet's own measuring unit, the chain loop is a building block that regulates stitch gauge and row gauge. All crochet stitches are built of chain loops, which also show in a stitch's characteristic "chain tops". Tall stitches are sequences of chain loops that catch yarn-over loops in them, and the Tunisian return pass is a sequence of chain loops that catch forward pass loops in them.  A slip stitch is a chain loop that has caught another loop in it of any type, of any stitch, of any material, in any direction. In this sense the slip stitch is a microcosm of the crochet macrocosm. Crocheting is the only known textile method in history that explores and expresses all of the chain loop's remarkable properties.  We use other loops in crochet too, but it's chain loops that "hook them up", space them apart at regular intervals, prevent previous loops from unraveling, and many more classic crochet functions.