Although I didn’t set out to write a book about a particular sweater construction, a good number of the sweaters in Plain & Simple are built on a hybrid construction of seamlessness. They start at the bottom and are knitted to the armhole. An inch or so of stitches are added at each armhole to extend it down the arm, then the piece is worked straight to the shoulder. Once the shoulders are joined using the three-needle bind off, stitches for the sleeves are picked up around the armhole and knitted to the cuff.
Why extend the armhole down the shoulder? Good question. What I like about this method is the smooth, uninterrupted shoulder line it yields, how it lets the knitted fabric curve gracefully over the bend. This adds a little je ne sais quoi to a plain sweater.
Oak from Plain & Simple, shown in Owl, color Papuan
In general, when you work a drop-shoulder sweater, a rectangle that begins at the hem and is worked straight to the shoulder without interruption (below), the shoulder line hits wherever the side seam takes it. Not always the most flattering place.
Cut in an inch or more, as in a modified drop shoulder, and once again, the shoulder line ends up somewhere near the shoulder but not at it and not necessarily somewhere that makes sense—just somewhere off the shoulder.
However, if you knit to the underarm and cast on some stitches, you take the seam down the arm, and this allows the shoulders and upper arms of the body (yours, not the sweater’s) to smoothly present themselves. And the taut curve at the underarm allows for ease of movement and arm independence. When you lift or otherwise move your arm, you don't drag the rest of the sweater with you.
In the sweater below (Ash), the long graceful line over the shoulder is accented with an exposed seam.
Instructions in the book call for using the working yarn to cast on an inch or more at the underarms when it’s time to separate front and back, then picking up stitches in those cast on stitches for the facing piece. Seamless, yes.
But you can also cast on those extra stitches with waste yarn, and when it’s time to work the opposite piece, simply unravel the waste yarn and place the live stitches on your needle and carry on.
Either way, you get a seamless underarm seam.