This pattern uses half double crochets, but it also requires you to cross them. Which is easy enough, you simply insert your hook into a stitch, half double crochet, then half double crochet into the stitch behind your first double crochet. The pattern will explain this as well. Don't be scared off by the fact that the pattern requires an intermediate skill level. Cross stitching in this way is one of the first things I tackled when I still considered myself at an easy level (and I'm still judging myself to be at an intermediate level in skill) and I had no trouble with it whatsoever. I'm sure you can do it too!