Different than mattress stitch, where two pieces of parallel knit fabrics are sewn together with the same number of rows, seaming perpendicular pieces of knitting together requires lining up stitches with rows and sewing them together.
While much of the actual seaming steps are the same as mattress stitch, the trickiest part is making sure you're joining the pieces evenly. A similar concept to perfectly picked up stitches, the key is making sure the length of the horizontal and width of the perpendicular pieces being seamed together are the same measurement, and calculating the ratio of rows to stitches. Below is a photo tutorial on expertly working this technique! - KGO
If you’re working from a pattern, the full depth of the armhole (bottom starting point up to shoulder, then down to the opposite bottom point) should equal the full width of the sleeve. It is easier to seam sweater pieces together prior to seaming the side and sleeve seams.
STEP 1 / Once your pieces have been blocked, double check that the length of the horizontal and width of the perpendicular pieces being seamed together are the same measurement. For my swatches, the horizontal piece is the teal swatch and the perpendicular piece is charcoal swatch, and the edges to be seamed together measure 5.5”.
STEP 2 / Count the row gauge of the horizontal piece and stitch gauge of the perpendicular piece. My horizontal gauge is 24 rows/4”, and the perpendicular gauge is 20 stitches/4”.
Calculate the ratio of rows to stitches (S/R) of the two pieces to the smallest fraction. (You will be using the same method as you would for evenly picking up stitches). Since my S/R is 20/24, the smallest fraction for my swatches is 5/6. In order to evenly seam the two pieces, I will need to seam 5 stitches of my perpendicular piece to 6 rows of my horizontal piece.
Note: This number may be different for you depending on your row and stitch gauge.
STEP 3a / Thread your seaming yarn (if the yarn is bulky, I recommend a smooth yarn in a similar color and finer gauge) onto a blunt tip darning needle. Insert the needle in and out of one stitch of the piece held perpendicular.
Step 3b / Insert the needle from front to back in the open space between the 1st and 2nd stitch on the first row of the horizontal piece, then bring the needle to the front in the second row.
Step 4 / Repeat this process, moving up one stitch on the perpendicular piece and one row on the horizontal piece until you have worked 1 less than your row ration number. Since my R = 5, I have worked 4 rows.
Step 5 / On the next row, insert the needle in and out of one stitch of the perpendicular piece as usual. Then, on the horizontal piece, insert the needle from front to back in the open space between the next row of the horizontal piece, then bring the needle to the front two rows up. You have now worked an additional row. For my swatches, this means I have worked 5 stitches and 6 rows, working the correct ratio.
Step 6 / Repeat Steps 4 and 5 until the full length of the seam has been worked. Once complete, the seam should not pucker in any way, and the pieces will evenly lie flat.
• Unlike seaming mattress stitch, where you can go 1-2" before tightening, I found it is necessary to tighten the seaming yarn after just a few stitches/rows.
• Make sure you stay in one column on the horizontal piece, and on one row on the perpendicular piece so your seam is perfectly straight.
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