Start at the shoulder
Check where the seam sits and how the sleeve falls when your arms rest naturally. Some shapes are deliberately broad or dropped, so compare the result with the intended fit rather than a single universal rule. What matters functionally is whether the upper back and arm allow the movements you need.
Look at the hem together
Try the jacket with the trousers or skirt you expect to wear most often. The point where the upper layer ends changes the balance of the whole outfit. Leave the jacket open, then fasten it. Both views are useful because they reveal different lines through the front.
Include the middle layer
A jacket fitted over a thin shirt may feel different over knitwear. Try the actual thickness you plan to use and check the cuffs do not bunch. If you alternate between light and substantial layers, decide which situation matters most instead of expecting one close fit to serve both perfectly.
Check the pockets in use
Put your hands in the pockets and see whether they sit naturally. Carrying a heavy phone in a light pocket can alter the front of the garment. Small habits affect how clothing behaves, and a fitting is the right moment to notice them.
