But one day I realized that "M1" is really just the pattern's shorthand, and each designer specifies a different stitch. In other words, it isn't like "PM" which always means "place marker," or "k2tog" which always means "knit 2 stitches together."
There is a lot of confusion on this point. At least once a month I see a knitter somewhere (on a forum or in real life) inform other knitters that "M1 means you knit into the front and the back of the stitch," or "M1 means you lift up the bar between stitches and knit into it." But this just isn't true. M1 means whatever the designer wants it to mean, so you'd better read that pattern!
The real problem arises when you start putting together your own knitting patterns for simple items. If you run across a place where you need to increase, it can be tough to choose the right one!
YO is almost never the right answer. Just to get that out of the way ahead of time. Unless you are knitting a lace pattern, or you specifically want the big holes as a design element. (The swatch pictured above uses a line of YO increases to make a pretty raglan line.)
The "backwards loop cast on a few extra" is also not my favorite. The cast on stitches are always wonky, and stretch badly, and look just awful. They also don't seem very stable, and are impossible to seam. I only use this increase method if the pattern absolutely demands it.
The "knitted on" increase is a good answer for a place where you need the stitches to be firm and inelastic. A buttonhole comes to mind. Or when you cast on extra stitches across an armpit (armscye) or the thumb of a glove.
KFB is one of my personal favorites. Although some people don't like the way it looks. Each KFB stitch has a little diagonal bar at a 45 degree angle. This can look nice, like a little stitched design element. But if you want an invisible increase, KFB isn't the right choice.
"Lift the bar" is a reasonably invisible increase. Although I find that as the garment is worn, it can stretch out the hole and make it a lot more apparent. Twisting the stitch by knitting into the back leg of the lifted bar can help with this.
"Lift the stitch below" comes in right and left-leaning varieties, and is my personal favorite. This increase can be a little fiddly, and it requires you to remember which one you're working (on the left or on the right). But it is the only truly invisible increase that I have found to date.
One nice thing about using this increase is that, because it comes in a pair, you can match two halves of a garment exactly. This is great for something like a raglan sweater!
Photo credit: Flickr/stitches1975