It's very common to decrease by knitting two together, but there is also such a thing as knitting 2 together through the back loop, which is often abbreviated k2tog tbl.
To perform this decrease, you work the same as if you were knitting through the back loop of just one stitch, but instead you go through two. Your right-hand needle goes through the back loop of the first stitch, then the second. Then make a knit stitch as normal, pulling both loops off the left-hand needle. One stitch remains on the right-hand needle.