Bracketing Blazers When the Shoulder Sits Between Sizes
Most blazer returns are argued about at the button. The button is not where the decision lives. It lives two inches higher, at the point where the shoulder seam meets your own shoulder.
Bracket two sizes and look there first. Everything below that seam can be adjusted. The seam itself is the garment's skeleton, and skeletons do not tailor well.
Read the seam before anything else
Put the blazer on over the shirt you will actually wear with it. Find your shoulder bone with a finger, the hard point at the end of the shoulder. The seam should sit on or just inside that point.
If the seam droops past the bone, the whole jacket hangs off a frame that is too wide and the sleeve head wrinkles. If it sits well inside, you will see diagonal pulls toward the neck when you raise your arms. Neither fault disappears with wear.
Then check button pull and vents
Fasten the top button of a two button jacket. A small horizontal crease at the button is a warning, not an automatic fail, because waist suppression is the cheapest alteration there is. What you cannot fix is a vent that splays open at the back while you stand still.
Reach forward as if shaking hands, then as if driving. The right size lets you do both without the collar lifting off your neck. A blazer that only fits in a catalogue pose will spend its life on a hanger.
Sleeve and body length are the cheap problems
Sleeves should show about a centimetre of shirt cuff. Shortening or lengthening a plain sleeve is routine work, unless the jacket has working buttonholes at the cuff, which turn a simple job into an expensive one done from the shoulder.
Body length is harder. A jacket that ends mid seat versus covering the seat is a style and proportion choice you should settle in the bracket test, because hemming a blazer body changes the pocket and vent balance.
Which size to keep when both almost work
Keep the one with the correct shoulder and have the waist taken in. That order is not snobbery, it is arithmetic: a waist alteration is a fraction of the price of shoulder work, and the result looks made for you rather than made to fit.
If both shoulders are wrong in opposite directions, keep neither. A different cut from the same retailer, a slim versus a regular block, often solves what two sizes of the wrong block never will.
Related reading
These earlier guides go deeper on connected questions: Bracketing Winter Coats When Layers Change the Fit, Bracketing Running Shoes for Marathon Training, and Bracketing Trail Running Shoes for Wet Terrain.
Frequently asked questions
Can a tailor move a blazer shoulder seam?
Technically yes, practically rarely worth it. It means rebuilding the sleeve head and collar area, and the cost approaches the price of a mid range blazer. Buy the size whose shoulder already fits and alter the waist instead.
How much shirt cuff should show?
Around a centimetre for most people, a little more if you like a sharper look. Judge it with arms hanging naturally, not raised. Sleeve length is one of the cheapest alterations, so do not reject a good shoulder over sleeves alone.
Should a blazer button comfortably with no pull at all?
A faint crease at the fastened button is common and fixable with a small waist let out or take in adjustment in the other direction. Strong X shaped pulling, a gaping vent, and a collar standing off the neck together mean the size is wrong.
Is a slim cut just a smaller size?
No. A slim block narrows the shoulder and chest pattern itself, while a smaller regular size shortens everything proportionally. If regular sizes bracket badly at the shoulder, trying the slim block in your usual size is the smarter second order.