This article explains how to purl 2 stitches together using the SSP, or Slip-Slip-Purl technique.
Often, pattern writers will tell you to do a decrease, without specifying which one. You have to know which one is appropriate for the situation.
Occasionally, knitting patterns will tell you specifically to do an SSP.
The SSP decrease will result in a right-leaning decrease on the other side of the fabric.
If you are working in Stocking Stitch (alternating Knit 1 row and Purl 1 row), then the SSP decrease would typically be used only on a Purl row.
Alternatively, it could be used to provide some texture in the knitted fabric. It depends on the knitting pattern.
The SSP is similar in concept to the P2tog thru back loops, but it has a useful twist to it.
The SSP is also similar to its cousin, the SSK (slip slip knit).
How to do the SSP (Slip Slip Purl)
Your target, as usual, is the next two stitches on the left-hand needle (for most knitters).
The first step is to slip the first stitch KNITWISE onto the right-hand needle.
The second step is to slip the second stitch KNITWISE onto the right-hand needle.
This photo shows the two target stitches sitting on the right-hand needle.
The third step is to slip these two stitches back onto the left-hand needle PURLWISE.
The effect of these three steps is to simply twist the stitches.
Look closely at a typical stitch on the left-hand needle. The strand of the stitch into which you insert the needle is a bit closer to the tip of the left-hand needle. The strand of the stitch behind the needle is a bit further.
But in this photo, after the twist, the strand that is closest to the tip of the needle is behind the needle.
Now that the stitches are twisted, it is easier to insert the right-hand needle into those two stitches from behind.
In this photo, the left-hand needle is the top one, and the right-hand needle goes through the two stitches and points right.
Work a purl stitch, carefully, onto the right hand (new) needle.
The photo shows the part in the purl stitch where the yarn that has been wrapped is being pushed to the back. Notice that the right-hand needle has moved to its normal position.
And the SSP decrease is now complete.
Even though there are a few steps to this decrease, it is easy to learn.