The Armscye measurement, loosely defined, is the size of the hole you need to put into a bodice, shirt, doublet, etc in order for your arm to fit through. I suspect that many people guess on this point, and tend to guess large. For many periods of history, including the Elizabethan era, however, the armscye measure needs to be fairly precise to help create the long, unbroken torso seen in period artwork.
This measurement is much easier to take with assistance, as you will be working under your armpit for part of it. The Armscye measurement starts at the tip of the shoulder. If, for some reason, you don’t happen to be made of plastic with fairly obvious joints, the tip of your shoulder is roughly where the bones of the shoulder line up. If your shoulder is more rounded, you should be able to press in to your flesh and feel for the bit that doesn’t give, on top of the shoulder line. Start here.
A properly fitted armscye continues fairly straight down, following the join between the arm and the torso. At the armpit, the armscye line lies along the torso, slightly below the armpit itself. (Exactly how far “slightly below” is depends on the period. For general fitting purposes, however, I’d recommend 2-3 finger widths below the armpit for a good fit that doesn’t feel hopelessly constricted. That should be 1.25-2 inches for those of you who don’t typically measure things off your hands. When working with a client, I generally put my hand on her side 2-3 fingers below her armpit, lower her arm, and ask her if she’s comfortable with a bodice/corset that sits right just here. Dolls, obviously, don’t complain too much about how things fit…) If you have particularly fleshy upper arms, you might find it more comfortable and more flattering to drop the armscye line further. I usually start with it higher, because it is far easier to alter a mockup by taking fabric out than by trying to put more fabric in. ;)
The Armscye should be measured along this line from shoulder tip, down next to the arm/torso join, in a smooth curve to 2-3 fingers below the armpit, then in a smooth curve back to the shoulder point. The join between the arm and torso is less clearly delineated at the back of the figure, what with how there’s a shoulder blade and everything seems to just flow smoothly. Just do your best when you’re measuring the armscye – out of everything, it seems that armscyes fluctuate the most for fashion (even more than the location of the fashionable waist!). What you’re establishing here is a minimum measurement – any smaller than this, and your arm won’t fit through the armscye.
As with several other measurements, it’s often useful to know how much of the armscye is in the front, and how much is in the back. Unfortunately, nature more or less defies pattern makers at every turn when it comes to the female torso. If you carry weight in your bust or in your arms, your armscye is probably not symmetrical. This time, since we normally start the measurement at the shoulder point, you just need to read the measure on the tape at the side line of the body – directly below your armpit. (Since you’re working right up in there anyway, sighting this should be a breeze…) The measure on the tape at the sideline is the Front Armscye measurement. The total measurement on the tape, minus the Front Armscye measurement, is the Back Armscye Measurement.