Interlock can be worked plain or in pattern, and results in a visually subtle edge that is highly elastic and does not look scalloped when unstretched. When worked in rib, it sits atop the last row and blends nicely into the fabric, following the bends of the ribbing. When worked plain over stockinette, it is the exact match for the Twisted (a.k.a. Backwards) Loop Cast On. An alternate deconstruction of the backwards loop cast on by my esteemed fellow knitter Lana Holden can be seen on this Ravelry thread.

THE CONCEPT
Let’s say you’re working stockinette stitch, and you pull out your knitting needle from a row of live stitches. Those stitches would look something like this:

Now, if each stitch were interlocked with its neighboring stitches, it would look like something like this: This is the end result of the Interlock Bindoff worked plain (see Part 1 below).

Applying this to k1p1 fabric, you would get a result that looks something like this (see Part 2 below):
The blue highlight illustrates the columns of knit stitches.

Instructions are for working in the round, because this bindoff is well-suited to socks, hats, and other garments that need a stretchy bound-off edge. If you are working flat, skip the Setup and start directly with Binding Off.

PART 1: INTERLOCK BINDOFF WORKED PLAIN

Worked plain over a row of stockinette, the Interlock Bindoff is the exact equivalent to the backwards or twisted loop cast on (with next row knitted through the front loop), making it an ideal choice for scarves, fingerless gloves, or any project that benefits from matched edges. In this picture, the pink shows the cast-on edge, and the red shows the bound-off edge.

Setup
Cut a tail of yarn at least 5 times the length/circumference of your edge, a bit longer if your edge is ribbed. (Note that the directions show a short tail in a different color – this is for illustration purposes only.) Thread the end through a blunt yarn needle (YN).

Then:
Draw the YN through the first stitch on your L needle as if to purl (back to front). Leave st on needle.

Draw the YN through the last stitch on your R needle as if to knit (front to back). Leave st on needle.

Binding Off
Insert your YN through the first stitch as if to knit, then the second stitch as if to purl. Slip the first st off the needle; leave the second stitch on the needle. Don’t pull out the slack yet!

Note: your working yarn is now in front. Bring the YN through the loop of working yarn before your first stitch (the one you just slipped off). Make sure this loop is not twisted, and go through it from front to back.

Snug very gently by pulling the YN horizontally to the right.

That’s all there is to it. Repeat steps 1 and 2, then gently snug, until all stitches are bound off. After binding off a few more stitches, it will begin to resemble the photo. Use consistent tension as you snug each stitch. It might take a little practice at first to get the tension right.

Finishing the Round
When you come to the last 2 sts, you’ll need to connect to the beginning of the round so you don’t have a gap.

After you insert your YN through the last 2 sts on your L needle, slip them both off together.

Draw the YN through the loop of working yarn and snug, as you did with all the previous sts.

Draw the YN through the very first loop of working yarn you made, just after Setup. Go through this loop from front to back.

Snug gently, then weave in your tail on the inside.

PART 2: INTERLOCK BINDOFF WORKED IN PATTERN

The Interlock Bindoff is excellent for ribbed fabrics, because it can be just as elastic as the fabric itself, and snaps back very nicely. Instructions below are for k1p1 rib.

If you understand the principles of grafting or duplicate stitch, you can adapt the Interlock Bindoff for use with any knit-purl pattern.

Setup
Start with a knit stitch on the tip of your L needle. Work setup as you would for plain Interlock.

Binding off
(1st st knit; 2nd st purl)
Insert your YN through both of the first two sts on the L needle as if to knit. Slip the first stitch off (the knit stitch). Leave second stitch on (the purl stitch).

Note that your working yarn is now in back.

Bring your YN through the loop of yarn before the k st you just slipped off. Make sure the loop is not twisted, and go through it from back to front.

Snug very gently by pulling the YN horizontally to the right.

(1st st purl; 2nd st knit)
With the YN starting in back, insert through both the first two sts on the L needle as if to purl. Slip the first stitch off (the purl stitch). Leave second stitch on (the knit stitch).

Note that your working yarn is now in front.

Bring your YN through the loop of yarn before the p st you just slipped off. Make sure the loop is not twisted, and go through it from front to back.

Snug very gently by pulling the YN horizontally to the right and repeat these 4 steps until all the sts are bound off.

Complete the round as you would for plain Interlock, except you will go through both the last 2 sts as if to knit.

If you lose track of the rhythm, pause. This mantra can help you get re-oriented:

• If first st is a knit st: go through first 2 sts as if to knit.
  Yarn will go to back: bring YN through loop from back to front.
• If first st is a purl st: go through first 2 sts as if to purl.
  Yarn will come to front: bring YN through loop from front to back.