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When sewing for costuming/cosplay, most of the patterns I tend to turn to have a "flat" style sleeve construction; that is, the front and back pieces of the garment are joined at the shoulder, then the sleeve is attached while both parts are still able to be laid flat, before closing both the sleeve and sides in one long, single seam.

However, the pattern I'm currently working with calls for a "set-in" style of sleeve construction--the side seam of the garment is closed up first, as is the sleeve seam, and then the finished sleeve is inserted into the armscye and eased into place before being sewn.

This raised a question for me of what the difference between the styles actually is; I find it easier to sew a flat sleeve, so is there a reason I should do the more time-consuming set-in sleeve instead? A search of some blogs and forums resulted in mixed responses; some said "there's no difference, do whichever you like," while others espoused the superiority of one or the other technique without any reasoning to back up their assertion.

So my question is simply this: What is the real advantage (or disadvantage) of a set-in sleeve vs a flat sleeve, if any?

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To be absolutely honest, the difference is marginal in most (but not all) cases.

You probably won't even notice any difference in:

  • short sleeves
  • wide or baggy garments (like a tunic or pirate blouse)
  • garments where the shoulder seam extends far down the shoulder (like a kimono or most casual menswear)
  • garments sewn from stretchy fabric

Puffy sleeves that are gathered at the top of the shoulder should always be set-in sleeves because that allows you to place the gathers where they need to be. In theory you could gather the (open) sleeve first and still use the flat method, but I honestly find it easier to use the set-in method in this case.

In tight fitting garments that don't stretch (like formal business shirts or coats) a flat-sewn sleeve might be too tight to accommodate the shoulder. The shoulder seam usually sits on top of the shoulder and the sleeve points slightly down at an angle. But the human body doesn't have angles like that, there is still the round of the shoulder that has to fit in there:

torso with a blue straight line representing flat sleeve and a green curved line representing set-in sleeve

The picture is a tiny bit exaggerated to make the difference visible. (Original image source)

The blue line represents the flat-sewn sleeve. The fabric forms a straight tube without any ease, but the round top of the shoulder somehow has to fit in there. Bulky people will notice the difference more than skinny ones.

The green line represents the set-in sleeve. It was cut wider than the armscye on purpose, then was slightly gathered on top to shorten the seamline to the same length as the armscye, before it was set in. Since the sleeve cap is a curve, some parts of it are cut on the bias and can compensate the gathering without puckering. The result is not a straight tube, but one that has a bulge at the top to accommodate the shoulder.

Here are some tutorials for set-in sleeves that clearly show the too-wide sleeve being gathered into the armscye:

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  • My first reaction to the question was that the two types could have differences in wear and tear: a longer seem ..seems more prone to tearing than a shorter one (especially when it has to suffer multiple directional forces and torque, like of the arm and the torso), but I can also imagine seems to actually be stronger than the rest of the fabric. Do you think there will be any differences in 'structural integrity' between the two?
    – Joachim ♦
    Commented Mar 5, 2021 at 9:16
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    The length of the seam has no influence on wear and tear if you use modern materials. Again, this only applies to tight fitting garments: The set-in sleeve will have more ease, meaning you don't pull the fabric as much when moving your arm. But depending on the fabric it might be a bulkier seam, which might receive more friction from jackets and other garments. I can very well imagine a flat sleeve ripping open at the seam when you move your arm abruptly... and it will tuck at the armscye uncomfortably.
    – Elmy ♦
    Commented Mar 5, 2021 at 9:25
  • This is a great answer, but a general question has nagged me since this thread was created. The diagram shows that clothing designs create the sleeve seam by following the line of the torso to the top of the shoulder. That falls short of the widest point on the shoulder (when the arm is down). That puts the shoulder in the sleeve, which can create a lot of stress on the seam when the arm is lowered. (cont'd)
    – fixer1234
    Commented Sep 8, 2021 at 22:28
  • The line from the neck along the top of the shoulder could be extended to the widest point of the shoulder, and the torso line could pivot out at the armpit say 20° to 30° to meet it. That would put the shoulder in the torso portion and there would be much less stress on the sleeve. Is there a reason why that isn't done? Just curious.
    – fixer1234
    Commented Sep 8, 2021 at 22:28
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    @fixer1234 Yes, current fast fashion does extend the upper shoulder seam past the widest part of the shoulder and the side seam is wider as well. The result is a very lose garment without much definition (I call it "potato sack fashion"). Doing the same with dress shirt fabric would definitely not look like a dress shirt (because the fabric doesn't drape or stretch well). It might resemble a traditional Kimono (that also extends the shoulder seam past the natural apex of the shoulder). The current standards for dress shirts stem from form-fitting tailored fashion ca. 150 years ago.
    – Elmy ♦
    Commented Sep 9, 2021 at 4:28
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It also matters how much the sleeve cap is curved. A steeply curved sleeve cap needs to be set it, medium or low curves can be done with either method.

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