Seaming adds a multitude of benefits to your knitting - it provides structure at important locations on a garment, such as the side seams and armhole. Knitting garments in pieces and then seaming also enables you to do modifications on individual pieces rather than the whole garment at once.
I could go on, but the fact that a lot of our garment patterns all utilize seaming in some way is a pretty good indication of how we feel about it.
For this tutorial, each swatch is a different color, and my seaming yarn is a 3rd color to show the individual details, but in reality you’ll most likely be working with the same color and yarn for all three. If you’re knitting with something super bulky, I recommend using a seaming yarn of the same color but a smaller gauge. Single ply yarns might also benefit from a plied seaming yarn for added strength.
STEP 1 / Thread your seaming yarn onto a darning needle. I always seam with a separate piece of yarn. To determine the correct length, I cut a piece about double the length I am seaming.
To begin, place the darning needle from front to back in the open space between the 1st and 2nd stitch on the first row of your knitting. I like to start on the piece that will sit on the right.
STEP 2 / Bring the needle up to the front in the second row of your knitting. Make sure to stay along the same column of stitches (in between the 1st and 2nd stitch from the edge). Pull your working yarn thorough, leaving a 7” tail for weaving in.
STEP 3 / Repeat Steps 1-2 on the left piece that you are seaming. Pull the needle and yarn through, but do not pull too tightly - you should have about 3/4" - 1" space between the pieces.
STEP 4 / Place your darning needle into the right piece (front to back) in the same space that you came out of in Step 1. Bring the needle up to the front in the next row of your knitting.
STEP 5 / Repeat Step 4 in the left piece of knitting, making sure to not pull the seaming yarn too tightly.
STEP 6 / Repeat Steps 4-5 until you have worked about 1” up your knitting. Make sure to go into the same space that you worked previously, so you move up a full row after working each step two times. You can see in the image above how the seaming yarn goes in and out of each space twice.
STEP 7 / Holding both the tail and your needle end, slowly pull the seaming yarn tight. Once complete, your seam should lay flat and not buckle and your seaming yarn should not be visible.
Continue in this manner, working back and forth for about an inch, then pulling tight, until you reach the end of your work. Since I have the exact same number of rows on each piece (see Keys to Success: Counting Rows), and I’m seaming evenly u I will end with my top seam perfectly aligned.
Once complete, the seam should look neat on both inside and out. The fabric should lay flat and not buckle or curl. If need be, you can adjust the tension before weaving in the seaming ends - because you’ve seamed neatly and followed the “in and out” rules outlined above, the seaming yarn should be totally independent of the fabric and slide back and forth easily.
When teaching mattress stitch in classes, we actually make students pull out the yarn - woooosh! - after seaming their first two swatches together. Not only is it a great lesson in letting go, it also is a good way for them to check that they’ve followed the steps correctly and not accidentally looped the yarn around to create a knot or skipped stitches.