When joining two pieces of knitting that require a neat even seam, it can be difficult to achieve this by sewing the seam closed. The 3 needle cast off is an excellent technique for such circumstances.
When you have finished knitting the first piece, leave the stitches on a spare needle of the same size as used for knitting the item.
After completing the second piece to be joined, keep the stitches on the needle and keep the yarn attached. Hold the two pieces of the knitting together with the right sides facing and the points of the needles aligned.
With a third needle of the same size, knit into both the stitches on the front and on the back needle at same time. Wrap the yarn around the third needle and draw through both stitches and drop both loops off the left needle completing the knit stitch as usual.
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