How to Mattress Stitch Two Knitted Pieces Together

Put both pieces flat on the table with the knit side facing up and the two edges you plan to join sitting next to each other. Keep them the same way round. If you flip one over you will be seaming a knit edge to a purl edge and the join will not close cleanly.

Mattress stitch needs a knit-stitch edge on both sides, so stocking stitch, ribbing and cables are all fine. Garter and reverse stocking stitch need a different join. Give the pieces a quick block before you start if you want the edges to sit flat.

Pick both pieces up and pinch the two edges together between your thumbs, with the columns of stitches running vertically in front of you. Norman from NimbleNeedles works this way for the whole seam rather than leaving the pieces flat on the table.

There is a reason for it. Held up close to your face, the small horizontal bars you are about to sew through stand out clearly. Flat on a table they hide in the fabric and you end up guessing. Guessing is how bars get skipped.

Cut a length of yarn and thread it onto a blunt tapestry needle. The long cast-on tail works too if you left yourself enough of one. Use the same yarn the pieces are knitted in so the seam disappears into the fabric.

The blunt tip is the part that matters. A sharp point pierces the strands of the yarn and drags the fabric out of shape. A blunt one slides between them. NimbleNeedles uses a contrasting red yarn in the video so you can follow the path, which is not what you want on a real project.

Look at the right-hand piece and count in one full column of stitches from the edge. Between that first column and the second one runs a ladder of tiny horizontal bars, one for every row. Those bars are what you sew through.

This is the part people get wrong. Picking up the edge stitch itself leaves a thick lumpy line down the join that no amount of tugging will flatten. Go one column in, find the lowest bar sitting right above the cast-on, slide the needle under it and pull the yarn through. Leave a tail hanging.

Take the needle across to the left-hand piece and find the same ladder there, one column in from its edge. Go under the bar sitting at the same height as the one you just used on the right, and pull the yarn through.

Leave it loose. Do not tighten anything yet. The yarn should lie slack across the gap between the two pieces, because you need to be able to see where the next bar is and a tight thread hides it. Tension comes later and it comes all at once.

Go back to the right-hand piece and slide the needle under the bar directly above the one you last used. Pull through, swap to the left, and take the bar above there. Keep alternating, one bar from each side, always moving up by exactly one rung.

The seaming yarn now zigzags across the gap like the rungs of a ladder. Do not skip a bar and do not double back. A missed rung shows up later as a small pucker that you can only fix by pulling the whole seam out.

Once you have worked an inch or so, hold the fabric steady and pull the tail. The red rungs vanish into the fabric and the two edges snap together into one piece. This is the moment the stitch earns its reputation. Done right there is no ridge on the front and no visible line.

Carry on up the seam, tugging every inch or so rather than waiting until the very end. Stop pulling the moment the edges meet. If you keep going the seam buckles and gathers, and you fix that by massaging the fabric back out with your fingers.

Turn the work over and run a finger down the join. Mattress stitch leaves a narrow ridge on the wrong side. That is normal and it sits flat inside a sweater or a cushion cover, where nobody sees it.

How fat that ridge gets depends on how far in you seamed. One column in gives the thinnest ridge. Seaming two or three columns in makes a heavier one, which is worth knowing because it is a quiet way to take half an inch out of a garment that came out too wide. Trim and weave the tail in and you are done.