Standard advice for buying a polo shirt includes an instruction to get sleeves that hit mid-bicep, which is fairly universal. More personal, on the other hand, is the relationship of sleeve opening and underarm measurements to your actual arms. If you don't have guns like James Bond here there's a risk that your arms won't fill out your sleeves, and this is a place where a loose drape is seriously not a good look. Fortunately it's easily remedied by a tailor, who can open up the underarm seam and taper the sleeve opening to be reasonably snug. While you're at it you may need to adjust the underarm length to maintain the proper sleeve angle. For sleeves at least, err on the side of low and tight.

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