We've spent a while talking about decreases -- how about some increases for a while? Let's look at the general types, some of their advantages and disadvantages, and where they can and cannot be freely substituted. We'll also talk about how increases are specified within patterns, which is more often vague than the way decreases are specified.
I'm going to classify the basic single increases into three groups: those which are done between existing stitches, those which are done into a stitch in the current row, and those which are done into a stitch in the prior row. Within these groups, they behave fairly similarly, and in most cases you can freely substitute one member of the group for another member of the group without doing serious violence to your end result.
The easiest of the between-stitches increases is the YO. Of course, easy doesn't necessarily mean simple, or I wouldn't have written quite a bit about them already, but fundamentally, it's just a loop of yarn over the needle. In common with the other increases in this group, the YO makes 1 stitch out of 0, not using up any of the existing stitches. Unlike the other stitches in the group, it has no directionality once completed; it has an orientation as it sits on the needle, but lies flat once you've worked it in the following row. And, of course, it leaves quite a noticeable hole; for this reason, it's really not interchangeable with the other decreases in the group. Pattern instructions are generally quite clear when this increase is desired, and will call specifically for this or one of the British terms for specific varieties of YOs.
The remaining between-stitches increases are close variants of one another, and with a few minor caveats which I'll get to as I discuss each of them, may be freely substituted. Typically an instruction of "make 1" or "m1" is an indication that this type of increase is expected, and you may use whichever one of these you prefer. Each of these does have a left-leaning and a right-leaning version, and in situations where you're doing a series of paired increases (for instance, both sides of a sleeve, or either side of the shoulder join on a raglan), it's usually best to use them in mirrored pairs. There are three variants: the lifted bar, the backwards loop, and the twisted YO.
The lifted-bar M1 will probably be the most familiar to you; most people learn it quite early in their knitting careers. This increase is done by picking up the bar that connects the next stitch on the left needle to the stitch just worked, placing that bar on the left needle as if it were a stitch, and then working into it (in knit or in purl, as appropriate) so that it's twisted, preventing a hole. The left-leaning variety is performed by putting the bar on the needle in standard Western orientation and working into the back leg; for the right-leaning variety you put the bar on the needle in Eastern orientation and work into the front leg. Amy at KnittingHelp has some nice photos of these; they may be found at the third knitted diamond down from the top, and are called M1R/M1F and M1L/M1B respectively.
The backwards-loop M1 may also be familiar; Elizabeth Zimmermann was quite fond of it and recommended it frequently. This increase is done by simply making a backwards loop of the yarn and placing it on the needle, just like the backwards-loop or e-loop cast-on. For the left-leaning version, the yarn is twisted clockwise, so that the working yarn points away from you; for the right-leaning version, it's twisted counterclockwise, so that the working yarn points towards you. Again, KnittingHelp offers photos; the first and second diamonds on the same page mentioned above show these increases, labeled as M1A and M1T, respectively. The first diamond also shows what happens if you don't use mirrored increases -- it's a less dramatic effect than what you get with unmirrored decreases, but you can still tell a difference. These increases differ from the lifted-bar M1s in two respects: they use up a bit more yarn and pull a bit less on the neighboring stitches, and the twisted stitch at the bottom of the new column appears one row later. If you have issues with this increase leaving a hole, you might try substituting the lifted-bar M1 instead; because it "steals" its yarn from the stitches on either side, instead of being given a stitch worth of yarn to itself at the time it's created, it tends to leave a smaller footprint. Conversely, if you have issues with the lifted-bar M1 causing pulling or puckering, this one is a nice substitute that usually doesn't have that problem. The one-row difference usually doesn't cause an issue, except that if you're counting stitches to see how many rows you've worked, you'll want to remember whether the twisted stitch is part of the increase row, or part of the row before that.
The twisted-YO M1 shares some of the attributes of each of the other types, and is done by creating a YO (either backwards or forwards) on the current row, and then working into it so it's twisted on the following row; a forwards YO is worked through the back leg to create the left-leaning version, while a backwards YO is worked through the front leg to create the right-leaning variety. This increase feels like a slightly looser lifted-bar M1 when you're working it, but like the backwards-loop M1, the twisted stitch appears on the increase row; the amount of yarn used is in between the other types. I favor this one, finding it to strike a nice balance between ease of working and neatness in appearance, but because it's really not completed until the row following the increase, I use it with caution where the row immediately following the increases is a complicated one.
That covers the increases done between stitches; now, let's look at those done into an existing stitch. These generally fall into two groups: those where you work the same type of stitch into both the front and back, in some order, and those where you both knit and purl into the same stitch, in some order. Unlike the between-stitches increases, these increases use up 1 existing stitch to make 2 stitches. Many patterns are explicit about these types of increases, with them spelled out within the line of instructions or clearly indicated as an abbreviation, but others are more obscure. When you are told to increase "in" or "into" a stitch, generally this type of increase is intended; that's fairly clear, since it is done within a stitch. However, often but not always, a pattern instruction that says merely "inc 1" is also looking for this type of increase; this seems to be a little more prevalent with British authors. If a pattern uses both "m1" and "inc 1" then you can be fairly confident that the "inc 1" is looking for this type of increase; if it uses only "inc 1", then be careful. Often the stitch counts will provide a clue: if a pattern says "k1, inc 1, k to last 2 sts, inc 1, k1" then you know that the "inc 1" is intended to use up a stitch, and therefore must be this type; if it says the last stitch instead of the last 2, then they must mean for you to use a between-stitches increase, because if you used this type there wouldn't be a stitch left for the k1. And with that out of the way, let's start looking at the mechanics of the individual increases.
First, we'll look at the front-and-back increases, and let's start with a fundamental question: why front and back, instead of just working twice into the same part of the stitch? The answer is that you can't work twice into the front without an intervening step. If you knit into the front of a stitch, and then immediately knit into the front of it again, you actually undo the anchoring of the working yarn into the stitch, and end up with a single stitch that's just wrapped around the needle in the middle, like what you get when you do a double YO. When you work into the front and then the back, or the back and then the front, a loop of the existing stitch wraps around the working yarn and secures it, so you can make that separate second stitch. This bit of yarn from the existing stitch winds up wrapped around the base of the second stitch worked, forming a little horizontal bar of yarn on the knit side of the stitch (you can see a good photo of this on that same KnittingHelp page, 5th diamond down); intriguingly, it's always the 2nd stitch and always the knit side of it that shows the bar, whether you do front first or back first, and whether you do knits or purls. Because of this little bar of yarn, these increases are sometimes called bar increases; however, that's susceptible to confusion with the lifted-bar M1 increase, so I think it's probably better to avoid the term and just be specific with KFB or PFB.
KFB is by far the most common of these, and is done by knitting a stitch through the front leg but retaining it on the left needle, and then swinging the tip of the right needle around to the back and inserting it in the back leg to work the stitch a second time; this gives you one normal-looking knit stitch, and a second to its left with a little collar twisted right up around its base. If you work into the back first and then the front, you don't get the reverse of this -- you get the front stitch obviously twisted, and then the second stitch looking just about the same as with KFB. I mention this primarily so you'll realize this doesn't give you a mirror-image; I'm not sure when you'd actually use it, except possibly if you were increasing in a twisted rib. PFB is done similarly, working into the front and then swinging the needle around to work into the back; again, the back-leg-first version just leaves that first stitch twisted, rather than forming a mirror.
You may have gathered that I think mirroring is kind of a Big Deal with increases of this type. Actually, I think it's important in any situation where you've got increases in pairs -- things just look funny if the fabric isn't increasing symmetrically -- but with these increases, the mirrored option is non-obvious. When you increase within a stitch, you'll have one column that appears to be the continuation of the column the stitch was originally in, and one column that springs up on the row where you do the increase; with each of the 4 increases I just mentioned, even if you reverse the order of the front and the back, the continuing column will always be the one on the right, and the new column always the one on the left. One common way to adjust for this is to move the leftmost increase one column to the right -- for instance, if you want your increase points to be 2 stitches from the edge, on the right edge you'd do "k1, KFB", but then on the left edge, you'd do "KFB, k2", to put the increased column as the 3rd column in from both edges. That's not bad; you can tell a difference if you have stacked increases, but it's not as dramatic as it would be with decreases, so you can get away with it. A slightly improved option is to do KFB on one edge on the RS rows, and PFB on the other edge on the WS rows -- you might do "k1, KFB, k to end" on the RS, and then "p1, PFB, p to end" on the WS, for instance. PFB still puts the increased column to the left, but since you're working it on the WS, it'll appear to the right on the RS; there's a one-row offset, but if the edges aren't brought close together, the eye won't pick that out, and the slant of the columns within the fabric will match nicely.
Until quite recently, I'd have said that was as good as things were going to get. However, very clever knitter Jenn recently pointed out to me a way to do a truly mirrored KFB, where every single thing is switched around, and the nice untwisted continuing stitch ends up on the left and the newly created column ends up on the right. To do Jenn's Mirrored KFB, you slip the stitch knitwise, then replace it on the left needle in Eastern orientation; knit it through the front and let it go, and then pick up the left leg again (still in Eastern orientation), and knit it through the back. It's a perfect match to the KFB -- isn't she smart? (It's quite possible, even likely, that someone else had already thought this up -- but I've never seen it before, and Jenn figured it out independently, so I'm giving her credit, although I'll be happy to provide equal mention to anyone else who's also documented it.) With that to go on, I've worked out a Mirrored PFB to match: slip a stitch knitwise and return it to the left needle, then purl it through the front and let it go; pick up the left leg, which will be crossed over the front of the stitch, into Eastern orientation, and purl it in the back. So, there you have it -- nicely mirrored options to match each of these decreases. I'd suggest using these any time you're doing KFB/PFB in pairs.
Now let's turn to the other group of stitches that falls in this category -- those where you knit and purl into the same stitch, in some order. The major difference between these and the front-and-back variety is that instead of the existing stitch getting wrapped around the working yarn to form a bar, the working yarn gets wrapped around the top of the existing stitch, forming a little nub sort of like half a stitch worth of moss or seed stitch. Because of this, this increase is sometimes called a moss increase; referring yet again to that handy page at KnittingHelp, you can see this one on the third diamond counting up from the bottom. Despite the name, however, this probably isn't the best increase to use when working moss or seed stitch. That's because this increase tends to open up the stitch it's worked into, leading to a small hole under the increase point; I think it's really best suited to the one case where that's a real advantage, which is when you're working into a YO, to make a large hole -- there, it gives a cleaner, smoother result than working into the front and back (and, in fact, a knit and a purl in some order are almost always indicated when working into the two loops of a double-YO, which is structurally a single stitch). The double-increase version of it has some other uses, which I'll get into when we talk about double increases generally, but the single increase really has just that one place where it's clearly the superior choice. It doesn't matter much whether you knit first and purl second, or vice versa; it shifts the slant of the nub a tiny bit, but so little that it's probably going to be overwhelmed in the texture of the yarn anyway.
What if you combined these, and did, for instance, a purl into the front and then a knit into the back of the same stitch, or the other possible combinations? Well, you can do that, and it doesn't give a bad result. It's a lot like KFB/PFB, with whichever stitch you do first and its twist or lack of twist being the most clearly presented; you get both the working yarn wrapping around the top of the stitch and the stitch wrapping around the base of the second stitch, so the type of stitch and whether it's twisted or not is pretty well obscured. It might be a little snugger than KFB/PFB, which would be all to the good, but it's a bit more of a pain in the neck to work, so I'd suggest experimenting and seeing if you think it's worth the bother. As with the KFB/PFB, you can do this in a mirrored fashion, but I'm going to leave that one as an exercise for those of you who're curious.
Before I move on, let me now clarify something that I deliberately left a bit obscure when I was talking about pattern instructions that indicate an increase to be done into a stitch: which kind do you do? In almost all cases, you're going to want to pick the KFB/PFB type; as I went into above, the knit-and-purl sort is best suited to those cases where you want an open result. In addition, your instructions may indicate which to choose as between the KFB and PFB by telling you to increase knitwise or purlwise. I have never seen a pattern indicate a mirrored KFB/PFB or the combined form, but I think I've given you sufficient guidance to understand when those might be called for.
Finally, there's our third class of increases, those done into a stitch in the prior row. There are only two of these, and they are mirrors of one another, so there's not really any question of substitution within this group. To perform these increases, you pick up a stitch from the prior row and knit into it; for that reason, these are often called "lifted" increases, but make sure you don't confuse them with the "lifted bar" type of M1 increase. The first of these is done by working into the stitch immediately below the next stitch on your left needle; you will pick up the right leg of this, place it on the left needle, and knit into it, then let it go. It leans right, away from the column where it was done, and is usually called a right lifted increase, or RLI. The second lifted increase is done by working into the stitch immediately below the last one that has already been knit, which will be two rows below the last loop on your right needle; you pick up the left leg of this, place it on the back of the left needle, knit into it, and then let it go. It leans left, away from the column where it was done, and is called a left lifted increase, or LLI. Amy at KnittingHelp has photos of this also, on the 4th diamond from the top, but calls them KLR and KLL, respectively. Meg Swansen suggests a variation on these, which is to knit them so that the old stitch is twisted; it's tighter and counteracts the tendency for a hole to develop where a stitch has multiple stitches worked into it. These increases are the best match for single decreases; they have nearly as strong a lean, and an appearance of stitches emerging from the side of a column which complements the way they seem to disappear under a decrease. This is particularly true when the lifted stitch is twisted, and is part of why I like this variation even though in other places (like the "improved" SSK) twisted stitches make me twitch. Usually instructions will be clear about calling for these if it's the designer's intent, but because they don't affect the count of the stitches waiting to be worked, they can be substituted for the M1 increases if that seems desirable to you.
Next up: we've covered the instructions that are usually used to indicate a particular intended increase, but it'll be useful to talk a bit about those cases where no intent can be determined, and how the fabric you're working in can affect the appropriate choice of increase there. I'd also like to cover the double/multiple increases, but I'm not sure if that'll be the same post or a different one, and then after that, with both increases and decreases in our toolbox, maybe I'll spend a bit of time on increasing and decreasing in pattern, particularly in ribbing and seed stitch. Stay tuned!