Last time, we discussed the single knit decreases, and what happens to the stitches that creates the lean. This time, we're going to look at the single purl decreases, and the double decreases.
The simplest of the purl decreases is p2tog. The needle enters through the right side of the first stitch and emerges from the left side of the second stitch; the yarn is pulled out through the right side of the rightmost stitch, resulting in the rightmost stitch lying on top. This is, therefore, a left-leaning decrease. However, there's a catch! This decrease is usually worked on the WS of the fabric, and therefore, while it slants to the left on the side where it's worked, it slants to the right on the RS. It's most often used at one end of a WS row to balance a left-leaning decrease on the other end of a RS row.
A more complicated purl decrease is the SSP, which is done by slipping two stitches, one at a time, as if to knit (not as if to purl), putting them back on the left needle, and then purling them together through their back loops. Let's look at what that does. Slipping the stitches in this manner changes each of them to the open orientation, but leaves the leftmost on the left and the rightmost on the right. This means that you're still working through the right legs of the stitches when you purl them together through the back loops, so you aren't twisting them. You enter from the left side of the leftmost stitch, so the new stitch is pulled through the right stitch first, and then the left stitch, which leaves the left stitch on top, and therefore creates a right-leaning decrease; however, as with the p2tog, this stitch is usually worked on the WS, which leaves you with the appearance of a left-leaning decrease on the RS.
Another possible purl decrease is the p2tog-tbl. As with the SSP, this decrease is done by coming into the back loops of the stitches from the left, and produces a decrease that's right-leaning on the WS and left-leaning on the RS. However, because the stitches aren't reoriented before they're worked together, they are twisted; this decrease resembles SSP in the same way that k2tog-tbl resembles SSK.
Now, let's look at the double decreases. There are double-decrease variants on the single decreases, such as k3tog and SSSK; these lean the same way as the single-decrease varieties they resemble, although the lean is generally even more pronounced because more stitches are involved. Since we've already discussed the mechanics of the single decreases, there's no need to discuss these types of doubles. However, there are also a couple of centered double decreases, in which the outer stitches lean together into the middle stitch.
The first of these, slip2tog-knit-p2sso, or S2KP, produces a prominent single stitch in the center, with the other two stitches tucked neatly underneath it; let's look at how that works. The first step in this decrease is to place the needle into the first two stitches together knitwise, as if you were going to do a k2tog, and then slip them to the right needle. This twists the stitches around into the open orientation, but does it as a group; the rightmost stitch ends up further to the left, and vice versa. This is a definite contrast to the one-at-a-time slipping in SSK, for instance. The third stitch is simply knit. Now, the two slipped stitches are passed together over the new stitch; this means that the stitch which was originally on the right ends up in the middle of the stack, and the stitch that was originally in the middle ends up on top, and that's what gives you that prominent central stitch.
The other centered double decrease is slip-k2tog-psso, or SK2P (note the different position of the 2 in this abbreviation). In this decrease, the first stitch is slipped knitwise, putting it into the open orientation. Next, the second and third stitches are knit together, and just like a regular k2tog, this puts the leftmost stitch on top. Finally, the slipped stitch is passed over, which leaves it at the top of the stack. The center stitch is on the bottom, and recedes, giving this decrease a softer appearance than S2KP.
I hope these last couple of posts have given you some useful tools to understand what's going on with your knitting, and to assist you in knowing which way your decreases are going to lean. Next up, we'll discuss briefly how to choose which decrease to use when.