All the basic stitches in crochet have a front loop and back loop, which form a V shape at the top of the stitch. Half double crochet (hdc) is unique because it has an extra, 3rd loop. Learn how to find this 3rd loop, whether working in rows or in rounds, and see what happens when you work into it.
What makes the 3rd Loop: The yarn over in a half double crochet forms a diagonal line at the front, and a horizontal line at the back of the stitch. That horizontal line at the back is the 3rd loop.
Finding the 3rd Loop:
When you work in rows, you turn your work so the back side of the previous row is facing you. This is also true if you are working in rounds but turning your work after joining each round. In these situations, the third loop will be facing you.
When working in rounds without turning, the front of the the previous round is facing you. In this case, the third loop is at the back. You can find it by tilting the work towards you slightly and looking behind the front and back loops.
Why work into the 3rd Loop?
When you work into the third loop, the top and bottom loops of the stitch are pushed forward. This creates a pretty braid-like row of v shapes. Doing this on every row or round can produce a faux knit effect.
Note: the below video is in US terms of Hdc in Australia we use the term Htr, both are the same stitch.