The dreaded ssp
Of all the decreases, the ssp (slip slip purl) is the most troublesome. We would avoid it altogether if it wasn't for the fact that once in a while we need to decrease on the purl side of the fabric AND we want the decrease to appear to slant to the left neatly - with no twisted stitches - when viewed from the knit side ... a tall order!
There is a common way of achieving this - slip 2 stitches individually knitwise, slip them together back to left needle purlwise, purl together through back loops. While this method does the job, it's often unpleasant to work because each move requires the wrists to twist in a different direction. The finished appearance of the decrease is also not always satisfactory, because the stitches tend to get stretched out in the process.
Here are two techniques that achieve the same end, but with smaller and more comfortable motions, and often with tidier results to boot.
In a nutshell: Slip 2 stitches individually knitwise, slip them together back to left needle KNITWISE, purl together through FRONT loops.
First to clarify what we mean by knitwise and purlwise ... when we knit or slip stitches knitwise, the needles are in agreement. They approach the stitch from the same direction.
When we purl or slip stitches purlwise, the needles are in conflict. They approach the stitch from opposite directions.
So we slip first one stitch then the next one knitwise from left needle to right. Notice how the stitches have changed direction - on the left needle their right shoulders were forward, now the left shoulders are in front.
Maintaining needles in their knitwise position, insert the left needle into the 2 slipped stitches, removing them from the right needle. Notice that, in being slipped back knitwise, the stitches are now back in their original right-shoulder-forward postions, untwisted, but they've changed places.
Purl the 2 stitches together - through their front loops as usual.
The view from the knitside shows a tidy left-leaning decrease with no twisted stitches.
Another method that's possibly quicker, although a bit more unorthodox, is a slip-drop technique.
In a nutshell:
Bring the right needle behind the 1st stich and into the 2nd stitch purlwise, slip the 2nd stitch off the left needle; this will force the 1st stitch off also ... slip the left needle tip into it making sure that it stays in its right-shoulder-forward position, slip the 2nd stitch back onto the left needle purlwise, purl 2 together.
And here's what it looks like:
Right needle behind the 1st stich and into the 2nd stitch purlwise; slip the 2nd stitch (along with the 1st) off the left needle;
Slip the left needle tip into the dropped 1st stitch, making sure that it stays in its right-shoulder-forward position;
Slip the 2nd stitch back onto the left needle purlwise, and purl the 2 stitches together - through their front loops as usual.
Again, the view from the knit side shows a tidy left-leaning decrease with no twisted stitches.