Slip slip knit or SSK, as you'll see it written in most patterns, is a basic way to decrease stitches. It makes a decrease that slants to the left and is often paired with knit two together, which is a right-slanting decrease. It's an easy technique to learn for decreasing stitches. The basics of this stitch are also used in several other stitches as well. It is almost identical to a cast off method known as binding off.
Many beginning knitters use the binding off method to finish the early works. It's especially used as a way to cast off scarves. If you're new to knitting, learning this stitch will allow you to follow several patterns and help build your knitting confidence.
How to SSK
To execute this decrease, slip the first stitch as if to knit, slip the second stitch as if to knit, then slide the left-hand needle into the front part of both stitches and knit them together.
Some people also perform this decrease by slipping the first stitch as if to knit and the second stitch as if to purl, working them together through the front as with the standard decrease. This makes it look a little more like a knit 2 together decrease, so if you're mirroring decreases on a garment you might want to try it.
When to Use
If you plan on knitting more than scarves, baby blankets, and other rectangular projects, you will need to learn how to increase and decrease the stitches in your projects.
Decreasing stitches is one of the main ways to add shape to a knitted garment or project. As you progress in your knitting career you'll find that slip slip knit is one of the most common ways to decrease stitches. Other forms of decreasing will give you a different look and feel, but this method is so easy there's no reason not to use it—if that's what a pattern calls for.
Baby booties and socks will often use this slip slip knit technique when shaping the ankle part of the garment. Stuffed animals also often use this technique to help shape the animal parts.
To balance out the left-leaning slip slip knit, it is often paired with the knit two together (k2tog) knit, which leans to the right. Together, these two techniques help even out the stitches so they don't look crooked. These stitches are said to be mirrors of each other.
Now that you know how to perform this common technique, you also know how to slip slip purl, as it's the same basic stitch only with a purl instead of a knit. Now that you've mastered the SSK, you can go on to knit many more patterns. Decreasing is one of the only ways to change the shape of a garment so the more you practice, the better a knitter you'll be.