Half double crochet (abbreviated hdc) is a one of the basic crochet stitches.
At all crochet schemes chain symbol would appear as: T.
At the scheme below you can see a stitches of half double crochet.
Half double crochet instructions
Yarn over the hook and insert hook in the last stitch of the previous row under top two loops.
Catch the yarn.
Pull the yarn back through the loop. You should have three loops on your hook.
Yarn over the hook and draw your yarn through all three loops on the hook.
Repeat the process inserting hook in the following stitches of the previous row under top two loops.
Guide to Back Loop Half Double Crochet
Save this step by step tutorial as a reminder