The Basic Method: Mattress Stitch
Mattress stitch is the standard way to join shoulder seams because it creates an invisible seam that lies flat and looks like a continuation of the knit fabric. You work it from the right side of the fabric, picking up horizontal bars between stitches rather than going through the stitches themselves. This keeps the seam flexible and prevents it from becoming a stiff ridge across your shoulders.
To start, lay both pieces flat with the right sides facing you. Thread a tapestry needle with yarn that matches your knit (use the same yarn you knitted with, or a similar weight and color). Insert the needle under the horizontal bar between the first and second stitch on one piece, then cross to the other piece and insert under the corresponding bar. Go back and forth, picking up one bar from each side, pulling gently every few stitches to close the seam. The stitches should disappear into the fabric as you work.
The key to a neat mattress stitch seam is tension. Pull the yarn snug enough that the seam closes, but not so tight that it puckers the fabric or creates a visible line. If you pull too hard, the seam will draw in and your shoulders will be narrower than intended. If you pull too loosely, gaps will show between the pieces.
Key Takeaways
- Mattress stitch creates an invisible seam by picking up horizontal bars between stitches, worked from the right side of the fabric.
- You must work with matching yarn and pull gently every few stitches to keep the seam from puckering or leaving gaps.
- Backstitch is a faster alternative for seams that will be hidden, though it creates a thicker ridge and uses more yarn.
- Kitchener stitch (grafting) joins two live edges with no seam at all, making it ideal for seamless shoulders on some patterns.
- Practice on a swatch first so you can feel the right tension before working on your actual garment.
Preparing Your Pieces Before You Seam
Before you pick up your needle, make sure both shoulder pieces are the same width and that you have the same number of stitches on each side. Count the rows on each piece from the armhole up to the neckline. If one side is longer, you may have made a tension error or miscounted your decreases. Unraveling a few rows is faster than trying to hide an uneven seam later.
Lay the pieces on a flat surface with the right sides facing you. Pin them together at the start and end points so they do not shift while you work. If your yarn is dark and hard to see, use a lighter-colored yarn for the seaming and weave it in later—this is easier than squinting at dark yarn against dark fabric. Make sure your tapestry needle is large enough that the yarn passes through easily without splitting the fibers of your knit.
Working Mattress Stitch Step by Step
Start at the bottom of the seam (the armhole edge). Bring your needle up through the first stitch on the left piece, between the first and second column of stitches. This anchors your yarn. Then insert the needle into the corresponding spot on the right piece, going under the horizontal bar between the first and second stitch.
Come back out one stitch over (under the bar between the second and third stitch) on the right piece. Cross back to the left piece and insert under the bar one stitch over from where you came out. Continue this pattern—one bar on the left, one bar on the right, moving up the seam. Every three or four stitches, pull the yarn gently to close the gap. You should see the two edges draw together and the seam disappear into the fabric.
When you reach the top of the seam at the neckline, weave the yarn tail through the last few stitches on the wrong side and trim it. The seam should be completely invisible from the right side, with no ridge or line visible.
Backstitch for Hidden Seams
Backstitch is faster than mattress stitch but creates a visible seam ridge, so it works best on seams that will be hidden—like the inside of a shoulder or a seam you plan to cover with ribbing. It also uses more yarn and creates a less flexible seam, which can feel stiff on the shoulders of a fitted garment.
To backstitch, place the two pieces with right sides together (wrong sides facing out). Thread your needle and insert it through both layers from back to front, one stitch in from the edge. Bring the needle back through both layers one stitch to the left, then insert it again one stitch to the right of where you just came out. This creates a solid line of stitches that holds the pieces firmly together. Pull evenly so the seam does not pucker.
Backstitch works well for joining pieces that will be seamed again (like when you seam the shoulder and then pick up stitches for a neckband), because the extra thickness gives you something solid to pick up into. However, for a visible seam on a garment where the shoulders show, mattress stitch will always look more professional.
Grafting Shoulders with Kitchener Stitch
If your pattern calls for Kitchener stitch (also called grafting), you are joining two pieces that still have live stitches on the needle—you do not bind off either piece. This creates a seam with no ridge at all, making it invisible from both sides. Kitchener stitch takes longer to learn than mattress stitch, but it produces the smoothest possible shoulder join.
To graft, hold both needles parallel with the right sides of the fabric facing you. Thread a tapestry needle with yarn. Insert the tapestry needle through the first stitch on the front needle from front to back, then through the first stitch on the back needle from back to front. Now work back and forth: insert through the front needle stitch from back to front (and slip it off), then through the next front stitch from front to back. Cross to the back needle, insert from front to back (and slip it off), then through the next back stitch from back to front. Repeat this pattern all the way across.
Kitchener stitch requires focus and a steady hand, but the result is a seam that looks like a row of knitting rather than a join. Many knitters find it worth the extra time for a garment where the shoulders are visible.
Common Mistakes and How to Fix Them
The most common mistake is pulling the seaming yarn too tight, which causes the seam to pucker and the shoulder to become narrower than intended. If this happens, you can loosen the seam by gently tugging on the yarn tails at each end, or by unraveling the seam and starting over with lighter tension. Practice on a swatch first so you can feel what "snug but not tight" actually feels like.
Another frequent error is picking up the wrong bar—going through the stitch itself instead of under the bar between stitches. This creates a visible line and a stiff seam. If you notice this happening, stop and look at where your needle is going. The bar should be a horizontal line of yarn between two columns of stitches, not the stitch itself.
Uneven seams usually mean one piece is longer than the other. If you notice this partway through, you can compensate by occasionally picking up two bars on the longer side instead of one, but it is better to check your piece lengths before you start. If the difference is more than a row or two, consider unraveling and reknitting one piece to match.
Choosing the Right Seam for Your Pattern
Most patterns specify which seam to use, but if yours does not, mattress stitch is the safest choice for shoulders because it is invisible and flexible. Use backstitch only if the seam will be hidden or if your pattern specifically calls for it. Reserve Kitchener stitch for patterns that leave stitches live on the needle, or for situations where you want a completely seamless join and have the time to work it carefully.
The weight of your yarn also matters. Lightweight yarns (fingering and sport weight) show seams less, so mattress stitch looks nearly perfect. Heavier yarns (bulky and super bulky) show seams more obviously, so some knitters prefer Kitchener stitch for those projects. With very thick yarn, even a perfect mattress stitch seam will be visible, so you might choose to make the seam a design feature by using a contrasting yarn color.
Frequently Asked Questions
What yarn should I use to seam my shoulders?
Use the same yarn you knitted with, or a yarn of the same weight and color. If you cannot find a perfect match, a slightly lighter shade is less noticeable than a darker one. Some knitters keep a ball of neutral gray or beige yarn on hand for seaming, since it blends with most colors.
Can I seam shoulders while the stitches are still on the needle?
Yes, if you use Kitchener stitch (grafting). This creates a seamless join. However, most patterns ask you to bind off first and then seam with mattress stitch, which is faster to learn and still produces an invisible seam.
What if my seam is puckering?
Puckering usually means you pulled the seaming yarn too tight. Loosen it by gently tugging the yarn tails at the ends of the seam, or unravel and redo it with lighter tension. Practice on a swatch to find the right tension before seaming your actual garment.
How do I hide the yarn tails after seaming?
Thread the tail through your tapestry needle and weave it through the stitches on the wrong side of the fabric for about two inches, then trim it close. The woven-in tail will not unravel because it is locked in place by the surrounding stitches.
Do I need to block my pieces before seaming?
Blocking before seaming helps may support both pieces are the same size and shape, which makes the seam sit evenly. It is especially helpful if one piece looks slightly different in width or length. Let the pieces dry completely before you seam them.