How to make a Half Double Crochet (hdc):
The half double crochet (hdc) falls between a single crochet (sc) and a double crochet (dc) in height. Instead of working off two loops at a time, you draw the yarn through three loops on the hook, so basically the half double crochet eliminates one step from the double crochet stitch and adds an additional wrap to the single crochet stitch.
Much like the single crochet it creates a fairly tight/dense fabric.
Wrap the yarn around the hook, then insert the hook onto the stitch.
Wrap the yarn around the hook, and draw the yarn through the first loop on the hook. Wrap the yarn around the hook again.
Draw the yarn through all three loops on the hook. One half double crochet made.