How to: cast off in rib pattern
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For a vertical pattern, such as a rib, you may see the instruction to cast off in pattern. This will ensure that the cast-off edge is consistent with the pattern - in the case of rib keeping the stretch - and producing a better-looking finish. By contrast, a rib section that is cast off knitwise would have a flattened look with a narrower profile and reduced stretch at the edge.
If you’ve not cast off before, do have a look at the useful casting off knitwise and purlwise first.
To cast off a 2x2 rib, you need two stitches to start with, so if your pattern starts with knit stitches, K2. Cast off the last stitch with working yarn at the back, by using your LH needle to lift the first stitch over the second one and off the RH needle. This leaves one st on the RH needle, and the first st has been cast off. The next st in the pattern is a purl st, so the yarn is brought to the front to purl the next st – always move the yarn between the needle tips to avoid an accidental yarn over increase. Move the yarn to the back, which also gets it out of the way, and cast off another st, use your LH needle to lift the first stitch over the second one and off the RH needle. The next st is purl, so bring the yarn to the front to purl the stitch, then once again to the back to cast off one st. This sets the pattern for casting off a 2x2 rib, which is very much like a regular cast off, except for changing your stitch as necessary and always moving your yarn to the back to cast off a stitch. Fasten off the last stitch in the normal way, by cutting the yarn and pulling the end through to neaten the stitch.