Men's Suiting
Suit jackets are sized by chest measurement plus a length letter — a 40R, a 42L — and that letter is not a minor detail. Short, regular and long change the jacket's body length, sleeve length and button placement together, so a 42L and a 42R are cut for two different torsos at the same chest size. Getting the chest right and the length letter wrong produces a jacket that measures correctly and still sits wrong through the whole silhouette.
The number no brand prints on the label is the drop: the difference the maker assumes between your chest and your waist, usually six to eight inches on a suit sold as a set. If your own difference is smaller or larger than the drop a brand builds in, the jacket and the trousers cannot both fit from the same set — which is the practical argument for buying separates, where the jacket and trousers are sized independently, rather than assuming a set will work.
The shoulder seam is the one fit point worth checking before anything else, because it is the one a tailor largely cannot fix: sleeves can be shortened, a waist can be taken in, a trouser hem is routine, but reworking a shoulder means rebuilding the jacket. Fabric affects how much give there is around all of it — a wool with a small percentage of stretch tolerates a closer fit than a rigid worsted at the same stated size.
Men's Suiting at retailers
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ASOS DESIGN tapered suit pants with linen in stone twill
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ASOS DESIGN boxy double breasted suit jacket in black
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ASOS DESIGN tapered suit pants in dark green
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ASOS DESIGN four way stretch skinny fit suit pants in navy
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ASOS DESIGN skinny suit jacket in dark green
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ASOS DESIGN slim suit jacket in blue twill
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ASOS DESIGN slim wool blend suit jacket in dark brown
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ASOS DESIGN slim suit jacket in green
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ASOS DESIGN slim herringbone suit jacket with wool in dark green
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ASOS DESIGN slim suit jacket in brown
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ASOS DESIGN slim suit pants in brown
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ASOS DESIGN double breasted regular suit jacket in brown micro texture
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ASOS DESIGN skinny suit pants in navy
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ASOS DESIGN wedding super skinny suit jacket in birdseye texture in dusky blue
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ASOS DESIGN wedding super skinny suit jacket in micro texture in grey
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ASOS DESIGN super skinny linen mix suit jacket in forest green
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ASOS DESIGN super skinny suit jacket in pastel lilac linen mix
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ASOS DESIGN slim suit jacket in charcoal
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ASOS DESIGN skinny suit jacket in black
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ASOS DESIGN skinny suit pants in stone linen rich