Have you ever looked at a sweater pattern and seen one of those sleeve charts, read about fitting the armscye, and turned away in fear? Well, you're not alone. Even experienced knitters can be intimidated by the thought of doing calculations and modifications. It is a high-reward process, though, because a well-fitted sleeve can take a garment from ok to fabulous. You can knit garments all your life and stick with basic pattern instructions or raglan sleeves. But there is no reason to shy away from trying a custom-fit, set-in sleeve.
First, the word itself isn't some fancy French term; it's just the word that has developed over time from the original phrase arm's eye. It means the arm hole. The armscye measurement refers to the entire perimeter of the arm hole on the body of the garment. The trick in making a set-in sleeve is to fit that armscye to the sleeve cap on the sleeve itself. Yes, most patterns usually give explicit instructions for a variety of sizes, but if you are making a garment by hand, there's no reason not to make it fit as well as it can.
Fortunately, there are resources out there to help you as you are actually working with needles and yarn. Craftsy's class The Seamless Artemisia Sweater has a segment on fitting a set-in sleeve. If you know how to knit, read a pattern, and can watch an internet video, you can do this. And that well-fitted garment will be worth it when you finish.
Creating the fit between the armscye and the sleeve cap, in a nutshell, comes down to:
Some knitters are extremely detailed and use trigonometry to make calculations. You don't have to go that far. Many knitters instead take a visual approach and use graph paper to create a map for knitting the sleeve cap and armhole. Or you can take a more casual approach and kind of wing it. How exactly you proceed will depend on your own comfort level. Be forewarned, if you go with the casual approach instead of graphing, you may find yourself having to rip out the sleeve cap a time or two.
The first step is always to measure your arm. (Or the arm of the intended recipient, but you deserve to knit yourself a sweater, so let's assume that's what you're doing.) You can't do anything until you know how many inches you need the armscye to be. Start with the measuring tape at the top of your shoulder, then loop the tape underneath your arm and back up around. That measurement will be your starting point. Of course, the actual armscye measurement of your garment should be a little bit bigger so your arm has room to move.
But beyond measuring your arm, try on some of your favorite shirts or sweaters. This is the best tip anyone ever gave me. Try to find one that has a fit like what you hope to achieve on your knitted garment. Then measure that armscye and work from that measurement.
Knowing your gauge is also important in this process. You don't have to knit a gauge swatch if you don't want to, but if you don't, you will need to measure your gauge as you work (before you get too far) and compare it to the pattern you're working from.
Once you know how many inches you want your armscye to be and how many stitches/rows of your project are in each inch, you can calculate how, if at all, you need to adjust the pattern to fit your arm. Your pattern should show you the armscye measurement for the different sizes. Keep in mind, that measurement shown on the pattern's schematic is the armscye depth, which represents half of the full measurement as it's only showing you either the front or back of the garment. So if your target armscye measurement is 16 inches, you're looking for the pattern measurement to be 8 inches. If the pattern offers plans for 7 3/4 inches and 8 1/4 inches, go to work adjusting the pattern a bit.
This is when you will break out the graph paper and make a chart of how exactly you're going to create the sleeve cap and corresponding body that will fit your arm perfectly. You now know you need to make an adjustment of a quarter of an inch. You know what your gauge is. You are equipped, then, to calculate how many rows you need to add or subtract and what adjustments you need to make in your decreases. Do you need to skip a row of decreases if you're adding rows or do an extra decrease for a row or two if you're subtracting rows?
You want to create a sleeve cap with a perimeter that matches your armscye. A good rule of thumb is that the sleeve cap height should be about 2/3 of the armscye depth. Because the sleeve cap height can be measured on a straight edge, it might be easier to set the top of your sleeve cap on your graph paper and figure out your rows and decreases from that fixed point to the fixed point of your first bind-off row at the start of the sleeve cap. Here is where the trigonometry can come into play as you want to make sure you create a shoulder slope that will get you to the right final perimeter measurement.
To measure the perimeter of your sleeve cap, you start at the beginning of the first bind-off row and follow the bell curve around to the end of that row. So if your armscye is 16 inches, you want that perimeter of your bell curve to be 16 inches. If they aren't exactly matching, you want the sleeve cap to be the longer measurement.