Crab stitch or reverse single crochet is a one of the basic crochet stitches.

At all crochet schemes crab stitch symbol would appear as:
At the scheme below you can see a stitches of crab stitch.
Crab stitch instructions
Instead of working from right to left, you work from left to right. Insert the hook in the last stitch of the previous row under top two loops. Pull the yarn back through the loop.
Insert hook in the last stitch of the previous row behind the loop. Pull the yarn back through the loop.
Catch the yarn and pull it back through the first and then the second loop.
You will see the first crab stitch.
Insert hook in the last stitch of the previous row under top two loops. Pull the yarn back through the loop.
Insert hook in the crab stitch under top two loops.
Catch the yarn and pull it back through the first and then the second loop. Then, pull the yarn back through the remaining loops.
Repeat the preceding steps in each stitch across the row.
Video tutorial
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