This stitch is a combination of 1 row in half double crochet and 1 row in slip stitch.

Starting with a wrong side row, work a slip stitch into your first stitch. You will not need a turning chain for this row because you will be working in slip stitches.

Work a slip stitch into each one of your stitches until the end of row.

Turn your work, you will now be working on your right side row. Make 2 chain stitches, these will be your turning chain.

Work a half double crochet into the next slip stitch. To make a half double crochet: wrap your yarn, insert into the slip stitch, wrap your yarn and pull through so there are 3 stitches on your hook, wrap your yarn around once more and pull through all 3 stitches.

Work a half double crochet into each slip stitch until the end of row.

Repeat these 2 rows and that will make up your half double crochet rib!