Front post and back post stitches, sometimes called relief stitches or cables, are common in crochet. You’ll most often see front post and back post double crochets or trebles, and sometimes also half double crochets. Front post and back post slip stitches, on the other hand, are much more rare, but they are easy and can be very useful!
One of my favorite uses for these stitches is as a “join as you go” technique to join work at a 90 degree angle. If you’re a knitter, think of it like the wrap and turn technique on a short row heel turning. In this example, I am using front and back post slip stitches to connect the instep of a slipper to the sides of the slipper. I need to connect the instep to the inside surface / wrong side of the slipper. This way, it creates a pretty little raised ridge along the outside of the seam, while leaving the inside surface nice and smooth.
This first row is a wrong side row, so I’m working a front post slip st (fp sl st). First, I insert the hook around the post of the stitch, from front to back and up to the front again, just like I would for any other front post stitch.
Then I yarn over and pull up a loop
Then pull that loop all the way through, just like I would for a normal slip st.
Easy, right? See how smooth the inside surface looks?
Now on the next row, I am working on the right side, so I want to work a back post slip stitch (bp sl st). First I insert the hook around the post of the stitch, from back to front, then out the back again, just like I would for any other back post stitch.
Then I finish it like I would for a normal slip stitch again: yo and pull up a loop, then pull that loop all the way through.
See? That was easy too! Now you can see how the little “V” from the top of the stitch you’re working around gets pushed to the outside surface. After a few more rows, here’s what the outside edge of that seam looks like. See the row of Vs along the seam? On the inside surface of the slipper, it feels and looks seamless.
This technique works well any time you are connecting the side of one row (here that’s the instep rows) to the top edge of another row (here that’s the side rows of the slipper). There are other, more decorative uses for this technique as well. It’s not a common one, but is great to know when you do need it!