In knitting, there are two main types of decreases involving two stitches – right-leaning and left-leaning. It’s really useful to be able to tell these apart and know which action will produce which result.
The first decrease most people learn is knit two together (k2tog). This results in the left-hand stitch on top, causing the stitch to lean to the right. The first stitch the needle enters is the one that ends on top.
Left-leaning decreases are a little bit more complicated. The two most common ones are slip one, knit one, pass slipped stitch over (skp) and slip one knitwise, slip one knitwise, knit two slipped stitches together through the back loop (ssk). In all cases, this results in the right-hand stitch on top.
Decreases can also be worked in purl and similarly, a p2tog is right-leaning and an ssp is left-leaning.
However, purl decreases are most commonly worked on the wrong side of the work. When you look at the back of a left-leaning increase, you also get a left-leaning increase, but it will point towards the opposite side of the work, which is something to bear in mind.