Knitting two together (abbreviated as K2tog ) is usually the very first decrease knitters learn. It's simple enough: you just knit the two stitches together as one. Sometimes that decrease happens with purl stitches, too. It's called a purl two together, abbreviated p2tog) Don't be stumped by this slight variation. It's just as easy as your trusty k2tog.
The purl two together is a nearly invisible decrease, thanks to all those purl bumps. If you know how to purl, then you can purl two together easily.
Here's how to purl two stitches together, plus what it looks like on both the knit and purl sides of your stockinette stitch.
How to purl two together (p2tog)
Work up to the two stitches that you want to purl together. If you're using a pattern, your pattern will let you know where the p2tog should go.
Bring the working yarn to the front of the work (if it isn't there already).
Insert the right needle into the next two stitches, as if you were going to purl them. That just means that the needle slides through the front of the stitches from right to left, with the knitting needle's tip pointing toward the left.
Wrap the working yarn around the needle in a counter-clockwise direction, just as you would when you're purling one stitch.
Pull the working yarn through both stitches. You'll be able to see the two stitches smooshed together just under the new loop.
Drop both of the old purl stitches off the left needle.
You have now decreased one stitch, turning two purls into one! So if you had, say, 10 stitches on your needle when you began, you'll now have 9 stitches.
The arrow in the photos above is pointing to that p2tog that I just made. It's practically invisible!
Purl two together in stockinette stitch
If you're working in stockinette stitch, it's likely your p2tog will be on the wrong (purl) side of the work. Flip that stockinette stitch over and you'll notice that the decrease created a stitch that leans to the right side.
Pictured above is the p2tog as seen from the knit side of the work. Notice how the stitches lean to the right on this side of the work, just like they would if you did a K2tog on the knit side of your stockinette stitch.
And here, pictured above, is the p2tog as seen from the purl side of the work. It's much harder to see the decrease on the purl side, isn't it? It's probably because those purl bumps are quite a bit more distracting that the neat little Vs of the knit stitches.
If you look at a p2tog in person, it's a bit easier to identify. When you pull apart the knitting, stretching it slightly on the purl side, you'll see the purl stitches stacked together, one right on top of the other. The part of the fabric with the p2tog also looks just a little bulkier than the other single purl stitches.
Where you'll find the purl two together decrease
This decrease can happen in any pattern when you need to decrease on the purl side, or just when you need to get rid of a purl stitch somehow. Perhaps that's why it's not super common; most of our stitching is done on the right side of the work, so we're more accustomed to k2tog and ssk (slip slip knit).
But there are certainly times when you'll see that sneaky p2tog. If you're knitting in k2, p2 rib, for example, and you need to decrease the purl columns, the pattern might ask you to k2, p2tog. Once you've made those decreases, that will result in a k2, p1 rib.
You may also see the p2tog decrease in lace knitting. I recently worked a lace border on a shawl, for example, that had yarn overs on the right side and p2tog on the wrong side to decrease those yarn overs.