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.
Repeat the step above so that there are two stitches on the right hand needle and then cast off a stitch as usual by passing the first stitch on the right needle over the second stitch.
Continue in this manner until there is one stitch left, cut the yarn and thread through the last stitch drawing tightly.
Here are views of the seams created from the wrong side and the right side:
This technique can also be used when you are creating a seam by folding a piece of knitting in half as in the MurisKnits “Flower Mittens” Pattern. This requires moving 1/2 of the stitches of the knitted piece on to a second needle and arranging the stitches so that when folded in half, the right sides of the knitting are together.
Pictured below is the top seam of a “Flower Mitten” which was completed using the 3 needle cast off method.