If you've ever knit through the back loop of a knitting stitch, you probably know that this produces a twisted stitch.
And though it is less common to do so, the same thing can be accomplished from the wrong side of a knitting project -- or indeed wherever a purl stitch falls -- by a move known as purling through the back loop.
How to Purl Through the Back Loop
Doing a purl through the back loop is a little more fiddly than knitting through the back loop to produce a twisted stitch, but it is not difficult to perform once you've tried it a couple of times.
Usually when purling you work with the front part of the stitch, the part that is facing you and closest to you on the needle. When you purl through the back loop, you do exactly the same steps as regular purling, but you work with the part of the loop that is on the back of the needle.
The key to doing this properly is to make sure you're entering the back of the stitch from left to right when working the stitch to get the right twist. You'll also sometimes see instructions that indicate going through the stitch from right to left, which still makes a twist, but I actually find it more awkward than the other method. As long as you're consistent it doesn't matter which technique you use.
Stitch Patterns Using Purl Through the Back Loop
This produces a twisted stitch and is used in the twisted stockinette stitch.
There's also a variation of single ribbing found in Barbara G. Walker's A Treasury of Knitting Patterns that involves knitting the knits through the back loop on the right side and purling the purls through the back loop on the wrong side throughout.
Or, to write that in knitting language (works on an even number of stitches):
Row 1: *K1 tbl, p1. Repeat from * across. (This is the right side)
Row 2: *K1, p1 tbl. Repeat from * across.
Repeat these 2 rows for pattern.
You can see a picture of this stitch in action on the Walker Treasury website.
Another Walker Treasury stitch that uses stitches purled through the back loop is called Twisted Check, which uses both knit and purl stitches worked in the back loops to make a sort of tweedy textured pattern.
Here's how that one goes (requires an odd number of stitches).
Row 1: K every stitch through the back loop.
Rows 2 and 4: *K1, p1 tbl. Repeat from *, end k1.
Row 3: *P1, k1 tbl. Repeate from *, end p1.
Row 5: Repeat row 1.
Row 6: *P1 tbl, k1. Repeat from *, end p1 tbl.
Row 7: *K1 tbl, p1. Repeat from *, end k1 tbl.
Row 8: Repeat row 6.
Repeat these 8 rows for pattern.
Using twisted stitches makes the fabric tighter, so if you're working a pattern that uses a lot of twisted stitches your gauge can be quite different from plain Stockinette. Make sure you always do a gauge swatch when working such a stitch pattern. You can also try working any pattern with twisted stitches on larger than called for needles
Also Known As: Purl into the back of the stitch, Ptbl, p1-b (in Barbara Walker)