Double Crochet
a taller, faster stitch than single crochet, used for garments, blankets, and open lace-like fabric

Double crochet is about twice the height of single crochet and works up faster, since each stitch covers more ground. It produces a softer, more open fabric with visible texture from the yarn overs, which is why it shows up in garments, blankets, and the granny square.
US "double crochet (dc)" is the same stitch as UK "treble crochet (tr)." One rung down, US "single crochet" matches UK "double crochet": every US stitch name sits one step behind its UK equivalent through the whole list.
How to work it
- Yarn over before inserting your hook (this is what makes it taller than single crochet).
- Insert your hook into the next stitch.
- Yarn over and pull up a loop. You now have 3 loops on your hook.
- Yarn over and pull through 2 loops. 2 loops remain.
- Yarn over and pull through the remaining 2 loops. One double crochet is complete, 1 loop remains on your hook.
- Repeat into the next stitch across the row.
- At the end of the row, chain 3 to turn. This turning chain counts as your first stitch of the next row, so skip the first actual stitch and work your next double crochet into the second one.
When to use it
Use double crochet when you want a project to move faster and drape more than single crochet allows: sweaters, scarves, blankets, and as the building block of the granny square. It's the standard "medium height" stitch most written patterns default to when they don't specify something more unusual.