The sleeve cap is probably the most confounding part of almost any garment. In fact, there is an argument that the interesting shoulder and sleeve details seen in recent couture collections are there in part to dissuade fast fashion copiers. There is some science and some art involved in making them work. The sleeve cap is a tricky and expensive place to make changes.
Any change to the body, armhole, chest, shoulder, and back, need to be examined for possible impact to sleeve caps. September 13 will have a post specifically on that. First, I think we need to just understand how sleeve caps interact with the body of the garment and the armhole.
Types of Sleeve Cap
Sleeve cap adjustments are very situational, so let’s look at the types of sleeve caps and armholes and how they work together.
The sleeve cap needs to allow the sleeve to fit into the armhole without ease (unless there is ease in the design) in knitwear. If you are a sewer you may be used to having ease.
This is really important: The total measurement of the sleeve cap where it gets sewn into the armhole needs to be nearly the same as the armhole where the sleeve gets sew into it.
Your sleeve cap can get wider and it can get taller/shorter. Think about these very basics of sleeve caps and armholes. Below I have illustrated each type of armhole. The front and back armholes are identical in these examples.
Flat Armhole Sleeve Cap
There isn’t actually a cap. The top of the sleeve should be exactly the same measurement as the armhole, front and back together.
Square Armhole Sleeve Cap
The sleeve cap here is exactly the height as the cut into the body.
Angled Cut-in Sleeve Cap
The sleeve cap height on the Angled Cut-in sleeve is the same as the depth cut into the body, and the angle is the same as the body.
Modified Set-in Sleeve Cap
The sleeve cap height on the Modified Set-in sleeve is the same as the depth of the armhole on the body, and the curve is created by working exactly the same decreases as the body.
Set-in Sleeve Cap
When you add a curved sleeve top for a set-in sleeve, the same concept applies, but now you have curves that need to fit together and curves that are going the opposite direction. The bind off needs to match on both the sleeve and body, but the decreases do not have to be exactly the same! Usually the first couple of decreases match, but after the initial curve the curve is going in different direction. The height of the sleeve cap will dictate how the sleeve hangs from the armhole. I find this easiest to consider if you start with the flat armhole.
The sleeve sticks out perpendicular to the body (90 degrees). This creates a lot of extra fabric at the underarm when the arm is brought down to be next to the body. A square armhole sticks out at 90 degrees as well, but now you have cut out a little fabric with the square. The idea is the more you cut out the less fabric you will have at the underarm. An angled cut-in and a modified set-in are the same thing, but the modified set-in a slightly refined version.
Cap height pushes the sleeve down from 90 degrees in a set-in sleeve. The higher the cap, the smaller the degree of angle from the body. So if 90 degrees is perpendicular, and all the fabric gets scrunched into the underarm, at 45 degrees, there will be less fabric at the underarm. Look at the Set-in Armhole, and imagine moving the sleeve so it fit into the armhole and you can start to see this.
However, now the garment will need to shift when the arm is lifted more than 45 degrees. So think a little bit about how you are going to move in a particular garment. Not many of us spend a lot of time with our arms perpendicular to our body, nor do we always have our arms next to our body (although that more often than straight out!). I like something about 45 degrees. I admit, I don’t have a formula, although one may exist, for mathematically figuring this out. I do it more by art, than science.