Let's talk about decreases! All single decreases, where two stitches become one, lean to one side, either the left or the right. Double (or greater) decreases, where three (or more) stitches become one, may lean to one side, or the disappearing stitches may lean into the center. What we're going to look at today is why the lean happens, and how to figure out and remember which decreases lean which direction.
Let me make it clear what "left leaning" and "right leaning" mean with respect to decreases. What this means is that the tops of the two (or more) existing stitches appear to lean to the specified direction. Therefore, for a left-leaning decrease, the tops of the two decreased stitches will be displaced to the left, with respect to the bases of those same stitches.
Also, this is one of the fairly rare posts where the manner in which you knit makes a difference: I am specifically discussing standard Western knitting (not combined), done with stitches beginning on the left needle and being worked onto the right. If you knit combined, or you knit from the right needle onto the left one, this discussion will not fully apply to the way you knit, and I'm not going to make it more convoluted by trying to specify how it's different. You may, however, still get something out of this -- I hope you will -- at least as a conceptual framework you can then apply to what you do.
If you don't know how to do some of the decreases I mention, or would just like to see some knitted-up examples or a video of how to do them, check out this page of decreases from KnittingHelp.com. Videos of the centered double decreases, but not knitted examples, can be found on the abbreviations page, listed as cdd and sk2p.
We'll start with the simplest decrease, k2tog. For a k2tog, you insert your needle in two stitches together, knitwise, and simply knit both as if they were one stitch. This is a right-leaning decrease. Let's look at what you actually do in detail, and it will become apparent why that is. When you put your needle into the two stitches, you are starting on the left side of the leftmost stitch, and the needle tip emerges from the right side of the rightmost stitch. Your working yarn is wrapped around the needle, and then drawn back through the two stitches. The end result is that the new stitch emerges from the left side of the leftmost stitch, and the two stitches are stacked up around the base of it, with the rightmost stitch underneath and the leftmost stitch on top of it. Because the new stitch wants to come straight up, this means that the tops of the prior two stitches are pushed to the right, and that's what gives you the right-leaning appearance.
Let's look at some of the left-leaning decreases. The simplest of these to execute is k2tog-tbl, where the needle is inserted through the back loops of two stitches together, and they're worked as one. In this case, the needle goes from the right side of the rightmost stitch and emerges from the left side of the leftmost stitch; the new stitch is pulled out the right side, and the stitches stack up with the leftmost one on the bottom and the rightmost one on top -- leaning to the left is exactly what you'd expect, and in fact what happens. This is also true for the SSK decrease, which is the same as k2tog-tbl except that the stitches are individually twisted first.
It looks rather like we're building up a rule where the direction the needle is inserted from gives you a decrease that slants in the opposite direction. But wait -- what about sl1-k1-psso (SKP)? There, you're slipping the first stitch, and knitting the second, and the needle is inserted from the left; shouldn't that give you a right-slanting decrease? Well, no. To see why, let's look at what actually happens to the individual stitches as you execute the SKP.
First, let me back up just a trifle and talk about stitch orientation a little bit. In normal Western knitting, stitches sit on the needle so that the leg further to the right is in front, and the leg further to the left is in back. I tend to think of the Western orientation as "closed", with the "face" of the stitch -- the part that will become the surface of the fabric -- turned away from the tip of the needle. The function of slipping a stitch knitwise, then, is to "open" it, turning its face toward the right; slipping it purlwise leaves it closed.
So, let's look at exactly what the mechanics of the SKP are. You slip the first stitch knitwise; that leaves it sitting on the right needle in the "open" orientation, with its left leg in front. Now, you knit the second stitch normally, which means you're going into the front, or through the right leg, as usual. You're coming in from the left, but you've only got one stitch involved, so it lies straight, not slanting either way. Now, you're going to take that first stitch, and pass it over the new stitch you just made. This gives us the reason for the "opening" step -- it allows the stitch to lie flat when passed over the next stitch. If it was still in the "closed" orientation, the right leg would cross over to the left side of the stitch, giving you a twisted top stitch, with a loopy appearance similar to that of k2tog-tbl (and the closed orientation of both stitches in k2tog-tbl is, of course, why it has that loopy appearance). Now, you've got two stacked stitches sitting around the base of the new stitch, and the right stitch is on top, so that if you consider the two decreased stitches as a group, the new stitch is emerging from the right side of the group. And that is the key to remembering which decreases lean which way -- they lean away from whichever stitch ends up on top.
Now, just as a point of interest, let's compare what happened with the SKP to what happens in an SSK. With the SKP, you ended up with the leftmost stitch face up, and the rightmost stitch, also face up, on top of it, around the base of the new stitch. When you do an SSK, you slip two stitches knitwise, one after the other. This opens each of them up, while keeping the one that was further to the right still on the right, and the one on the left still on the left. You then knit them together through the right legs, which are now in the back. The new stitch is drawn first through the leftmost stitch, and then through the rightmost stitch, so that they both end up laying face up around the base of the new stitch, with the rightmost stitch on top. Doesn't that sound familiar? That's right -- structurally, SSK and SKP are identical. So why do they sometimes look a little different? Execution. It's easy in an SKP to stretch out that passed-over stitch, which makes it a little looser and a little more prominent. Working on the tips of the needles, and concentrating on not tugging too much as you pass the stitch over, will improve your SKP.
Speaking of improvement, what about the so-called "improved" SSK, where the second stitch is slipped as to purl? That means that the second stitch stays in the closed position, which means it's worked through the left leg rather than the right one, and that twists it. This is why some people achieve a flatter result; the twisting tightens the lower stitch, which pulls on the upper stitch and keeps it neat and small. However, the twisted lower stitch is sometimes visible, particularly in very smooth yarns like mercerized cotton; I've even found it to sometimes cause a small hole under the decrease, when knitting a smooth yarn at a gauge that's tight for the yarn.
And, just as a point of interest, what about the knit-return-pass-return decrease, which is shown at KnittingHelp as another right-leaning decrease? As you may suspect, it bears the same relation to k2tog that SKP bears to SSK -- in other words, it's structurally identical, but the manner of execution tends to loosen the passed stitch a bit and make it a little more prominent.
I think that's actually enough for a single post. I'm going to stop here, and we'll pick up next time with a look at purl decreases, and double decreases.