One spring, I talked about how to make invisible seams on vertical stockinette stitch and garter stitch. Perfect for sewing the front to the back of a sweater. Remember that? No? Might want to check it out for "general principles of seaming" since I won't be repeating them this time. Except for "block first" since you really want to block first. Seriously. Block first.

But what if you want to sew the sleeves onto a garment? You've got a row of vertical stitches - the stitches on the body of the sweater - that will have to be matched up to a row of horizontal stitches Ð the ones on the sleeve, and that's a whole different can of worms.

Let's take a moment and consider stitch vs. row gauge. Grab your nearest pattern or yarn label and check out the gauge. The row gauge is nearly always a slightly higher number than the stitch gauge - something like 4 sts/6 rows = 1 inch or 16 sts/24 rows = 4 inches. In other words, there are more rows in each inch of knitting than there are stitches Ð therefore you can't just match up one sleeve stitch to one body stitch, because it won't come out right.

Basting

Before seaming, you might consider basting the sleeves onto the body of the garment to line the sleeves up just so and make sure it fits the way it should.

Place the right sides together. Using a contrasting (and thus easier to remove afterwards) length of yarn threaded on a blunt tapestry needle, insert the needle close to the edges, back and forth through both thicknesses.

(Basting could also be used as a way to seam, but it is neither stable nor attractive enough to recommend.)

Once the sleeves and body are basted, try it on. Does it fit the way you want? Are there puffy sleeves when there shouldn't be? Do the pieces lie flat and even? If so, attach the two peices together (pin or tie with small lengths of spare yarn, as below) at regular intervals to guide you when you're seaming, and remove the basting thread.

Backstitching

A somewhat better method of seaming is backstitching. The backstitch is also worked with the right sides together - much in the same way as basting - but overlaps by starting next stitch at middle of preceding one and thus makes a stronger, if a bit bulky, seam. To backstitch, you'll want to use either the yarn you've knitted the garment with or, if it's a thick yarn, a thinner one in a matching color. I've used a contrasting color yarn in the following pictures to make it easier to see what's happening.

Insert the needle through both thicknesses...

Then reinsert the needle near the point where the yarn went through from the previous stitch. Keep moving forward a couple of stitches at a time and try to keep the stitches even and straight.

Backstitching is be a good choice to use when you want to hide less than perfect selvedge stitches. It's also a good method for sewing sleeves into steeked armholes. Do the backstitching in the first "neat" stitch in from the edge. You might consider running a basting thread loosely through the knitting one more stitch in from where you'll want the backstitching to be to act as a guide if the stitches are difficult to see.

Invisible vertical to horizontal seaming

A better method of seaming is weaving. It's quite similar to mattress stitching, except that to seam vertical to horizontal you'll have to take into consideration the difference between stitch and row gauge.

Work back and forth, inserting the needle under the horizontal bar between two stitches on the vertical (body) side...

Then under a whole stitch on the horizontal (sleeve) side..

Now to compensate for the difference in stitch vs. row gauge: after repeating the previous two steps three times, insert the needle under 2 of the horizontal bars on the body side at one time.

Keep repeating the previous steps: *Under 1 horizontal bar on body, under a whole stitch on sleeve* three times, then under 2 horizontal bars on body, under a whole stitch on sleeve once, until you've completed several inches.

Then pull gently on the sewing thread to carefully tighten up the seam, so that it lies flat but doesn't pucker.

A few more thoughts about sewing in sleeves:

Use a length of yarn longer than the length of the seam. Start at the top of the shoulder and pull half of the total yarn length through and seam downwards towards the underarm. When you're through with one half of the seaming, begin again at the top of the shoulder with the other half of the yarn. You'll be more likely to be consistent and the yarn you're using to seam with won't get frayed by the time you're finished.

References:
Vogue Knitting
The Knitter's Book of Finishing Techniques