If you're a knitter, chances are you've come across the term purl 2 together through the back loop (or p2tbl). This is a decrease stitch that's worked over two stitches and lean to the left. It's a great way to create nice, even decreases when you're working on a project with symmetry (like a scarf or blanket). Here's how to do it:
To purl 2 together through the back loop, start by inserting your needle into the second stitch on your left-hand needle from behind (as if you were going to purl), then insert it into the first stitch on your left-hand needle from behind as well (so both needles are now in front of your work). You'll see that there's now a loop formed between the two needles. Wrap your yarn around the right-hand needle clockwise and pull it through both loops on the left-hand needle. Now you've decreased by one stitch!