StyleForum member tailorgod has a nice animation to illustrate what is meant when someone says that a jacket’s sleeve pitch is off. Sleeve pitch refers to the angle at which a sleeve is inserted into an armhole. If the curve and angle of the sleeve don’t harmonize with the way the wearer’s arms naturally hang, you’ll get unsightly bagging at the front or back of the sleeve (usually the back). There’s an angle that fits most men, but some people have slightly unusual posture. Military men, for example, often stand more erect, and older gentlemen can have a bit of a stoop. One tailor told me that he’s seeing younger people these days with the same curved shoulders, which he suspects is from being at a computer all day. In any case, if you have such posture issues, you may need to have your sleeves rehung.
To see if your sleeves hang correctly, just put your jacket on and stand sideways next to a mirror. If your sleeves look like this in the third photo, they’re perfect. If they hang like this, you may need to have them adjusted. How easy or complicated (read: cheap or expensive) this job will be depends on a number of issues, which are best addressed through your local tailor.
To be sure, sleeve pitch is probably the last thing one needs to worry about with off-the-rack clothing. Most men’s problems aren’t due to an incorrectly set sleeve, but rather a jacket that is too baggy or too slim. Or there are stylistic issues, such as the gorge or buttoning point being too high, or jacket being too short. Any of these are more likely to make you look bad in a suit or sport coat than an incorrectly pitched sleeve.
Still when trying on a suit or sport coat, it may be good to pay attention to yet another aspect of fit: whether or not the sleeves harmonize with the way your arms naturally hang.
(Illustrations by tailorgod. Thank you tailorgod)