There are several types of yarn ends that can occur in a knitting project. There is the tail from the cast-on; the tail from the bind off, and tails from any joining in of a new ball of yarn or color changing you will have to contend with. Each situation demands a different end-weaving technique.
When weaving in yarn ends, the important things to focus on are maintaining the elastic nature of the fabric, avoiding puckering, keeping the tail well hidden, and absolutely never using a knot. Knots have a tendency to work their way through to the right side of the fabric and show. They are also uncomfortable for the wearer.
Yarn Tails Along Side Seams It is always best to join in your yarn at a selvedge edge that is going to be seamed. If you are knitting in the round, joining in the yarn at a location that is less visible, such as under the arm, is a better place to join than on the front. When you are weaving in yarn along the seam, you can go ahead and darn it through the selvedge stitches. Simply thread the yarn tail with a darning needle and pull it through the bumps of your selvedge edge. When you seam the sweater, be sure that this edge is fully inside the garment. You can also weave in your cast-on and bind-off tails at seams.
When weaving in a cast-on or bind-off tail, be sure never to run it along an edge such as a collar or the edge of a sweater or vest. These stitches move around a lot, and the tail will likely work its way out.
Yarn Tails Within Knitting, What about color changes or joining in yarn in the middle of a round? When you are knitting make sure you leave a tail at least 6 inches long. When you are ready to weave in the end, be sure to adjust the stitches around the join so that the tension of the stitches is all even. Thread the end through a tapestry needle, and then weave it through the stitches it would have been a part of had the yarn piece continued on. You will want to try to match the surrounding stitches. In stockinette, on the wrong side you can run the thread through the top purl bump of each stitch. In garter stitch, try to weave the thread through the stitches in a way that matches. This is called duplicate stitch. You seam in duplicate stitch by sewing vís over the knit stitches to match the appearance of the knit stitch your seaming through. If you are going over a purl stitch, try to match its bump.
Yarn Tails In Unseamed Projects At The Cast-on or Bind-off Edge What do you do for that flat scarf, washcloth, or other piece of knitting that has a cast-on and bind-off tail and no seam to hide it in? You want to be sure to get the tail away from the edge, and then run it across vertically or horizontally, whichever is the most invisible. Begin by threading the tail through your darning needle. On the wrong side, run the stitch diagonally at a 45 degree angle into the fabric, using duplicate stitch for about 5 or 6 stitches. Then, go either vertically or horizontally using duplicate stich for the rest of the tail.
Whether to block before or after weaving in ends seems to be a matter of personal opinion. Some knitting references suggest that by weaving in ends and then blocking, your ends will also benefit from the final setting of the stitches. Other references state that if you weave in your ends before blocking then, the ends can lead to a puckering effect along the edge. Try weaving in one end of your original swatch and blocking it to see which way you prefer.
Readerís Digest Knitterís Handbook, Montse Stanley, Readerís Digest, Pleasantville, NY 1993.
Holladay, A. (2003). Those pesky yarn tails. Cast-On, 2003(Spring), 13-15.