If you're going to do any fancy decreases other than the basic k2tog (knit 2 together) or ssk (slip, slip, knit), eventually you'll bump into the skp (slip, knit, pass) decrease.
The slip knit pass is a left-leaning decrease that pops up often in lace knitting. It decreases your stitch count by just one stitch, and you just might find that it actually looks a lot like one of the basic decreases. (More on that later!)
You may also see this decrease written as sl 1, k1, psso (slip 1, knit 1, pass the slipped stitch over), and it's usually worked on the right side of the work.
No matter how this decrease presents itself, the process is the same. See the tutorial below, then keep scrolling to compare the skp to another decrease.
SKP (slip, knit, pass) tutorial
Step 1: Slip one stitch
Work up to the stitches that you want to decrease. (If you're using a pattern, the pattern will tell you where the decrease goes.)
Slip one stitch knitwise by inserting your right needle into the next stitch as if you are going to knit it...
...but instead of knitting the stitch, simply transfer it over to the right needle by slipping it off the left needle. You've now completed the first step of the skp: slip.
Step 2: Knit the next stitch
Knit the next stitch. You've now completed the second step of the skp: knit. (Easy!)
The first stitch on your right needle will now be that new knit stitch, and right next to the knit stitch will be the stitch you slipped. Keep your eye on that slipped stitch!
Step 3: Pass the slipped stitch over the knit stitch
Insert the left needle into the front of the slipped stitch (the second stitch on your right needle).
Using the left needle, pull up on the slipped stitch and pass it over the knit stitch and off the right needle.
Pull the left needle out of the stitch so that it's now completely off the needle. The slipped stitch now sits snuggled up next to your knit stitch, and you have one less stitch on your needles.
This action might feel familiar because it is similar to the action you use when you bind off, slipping the stitches over one another.
You've now completed the last step of the skp: pass.
After you work a few more rows, you'll see the skp. Check out how the stitch leans over to the left. Remind you of another decrease?
SKP vs. SSK
Both the ssk (slip, slip, knit) and the skp (slip, knit, pass) decreases result in one left-leaning decreased stitch.
If you look at both decreases, they look the same. In the photo above, for example, it's pretty difficult to tell the difference.
Sometimes is that my skp stands out just a little more than my ssk. I think that's because when I pull on the slipped stitch to pass it over, I just might be elongating the stitch a little bit more. That makes it look a little more prominent than the ssk. When I'm working the ssk, though, my stitches are closer together and a bit tighter when I do slip the stitch over. But that might not be the case for all knitters.
I've heard other knitters say they prefer the skp over ssk because there are four movements involved in ssk, while skp only involves three.
In the end, use whichever decrease you think looks the best and is the easiest for you to make!