Kitchener stitch can be used to join two pieces of knitting together. With practice, this technique can produce a seamless effect.

 

Place the stitches to be joined on two needles with the same number of stitches on each. Hold the needles parallel to each other with the points to the right. Using a yarn tail 3-4 times the length of area to be grafted, place the yarn tail on a yarn needle and

  1. Bring the needle through the first stitch on the front needle (a knit stitch) as if to purl. Leave the stitch on the needle.
  2. Bring the needle through the first stitch on the back needle as if to knit and leave the stitch on the needle.
  3. Bring the needle through the first stitch on the front needle as if to knit, then drop this stitch off the needle. Bring the needle through the next stitch on the front needle as if to purl, leaving the stitch on the needle.
  4. Bring the needle through the first stitch on the back needle as if to purl and drop this stitch off the back needle. Bring the needle through the next stitch on the back needle as if to knit, leaving the stitch on the needle.

Repeat steps three and four until no more stitches remain.

Note: There are many tutorials on the web that show this technique.