Quick Answer: A blazer’s shoulder is the structural foundation - the sleeve head, the canvas, the pad and the armhole are all built in relation to it, so changing it means rebuilding the jacket rather than altering it. Everything else is comparatively easy: sides, sleeves, length and even the waist are routine. So buy for the shoulder and alter the rest, and check the shoulder seam sits exactly at the edge of your shoulder bone with no divot beneath it.

The jacket that fits everywhere except the shoulder is not a jacket that fits.

And the one that fits only in the shoulder is very nearly finished.

Why is the shoulder structurally fixed?

The sleeve is set into the armhole with a precise distribution of ease, and any change to the shoulder changes that distribution.

Canvas and interfacing run through the shoulder and chest, so narrowing it means opening the jacket up to its internal structure.

Shoulder pads are positioned relative to the seam, and moving the seam means repositioning them and re-balancing both sides.

The armhole depth changes when the shoulder width changes, which alters how the whole jacket hangs from the body.

Which means the work is closer to remaking the jacket than altering it, and the cost reflects that.

A shoulder alteration on a good jacket can approach the cost of a new one, which is why tailors tend to talk people out of it.

What does a correct shoulder actually look like?

The shoulder seam sits exactly where your shoulder ends, at the bony point, not before it and not beyond it.

There should be no divot or dent beneath the seam. A dimple just below the shoulder means the seam is too far out.

No pulling across the upper back. Diagonal wrinkles from shoulder to armhole mean it is too narrow.

The sleeve should hang cleanly without ripples at the top, which is what a correctly set sleeve head produces.

Check it with your arms down and then raise them once. A jacket that binds immediately on raising the arms has an armhole cut too low, which is a separate but related fault.

And check it from behind if you can, since the back shoulder is where the fault is most visible and least examined.

What can be altered, and what does it cost?

Taking in the sides is straightforward and the single most transformative alteration for most people.

Shortening sleeves from the shoulder is skilled but routine, and it is the correct method on tailored jackets.

Shortening the body is possible but limited by pocket position and the vent, so there is less range than people assume.

Taking in the back seam sharpens the waist and is common on jackets bought for the shoulder.

Adding room is much harder than removing it, since it depends entirely on how much seam allowance the maker left.

Which gives a clear buying rule: buy for the shoulder, size up if you are between, and have the rest brought in.

Myth vs Fact: Blazer Fit

Myth: a slightly loose shoulder is fine because it reads as oversized.

Fact: intentionally oversized blazers are cut with a dropped shoulder as a design, with the sleeve head and armhole built for it. A regular blazer one size too big has the shoulder seam in the wrong place relative to its own construction, which is why it collapses rather than drapes. The two look nothing alike in person.

Myth: shoulder pads are outdated.

Fact: nearly every structured jacket has something in the shoulder, even if it is a thin sleeve head roll rather than a visible pad. The pad is what keeps the sleeve hanging cleanly. Removing them from a jacket that was built with them produces a collapsed shoulder, not a modern one.

Myth: if it buttons, it fits.

Fact: the button is the least informative test on a jacket. An X-shaped pull at the button means it is too tight through the chest; a jacket that buttons comfortably can still have a shoulder two centimetres too wide.

Myth: you should be able to raise your arms fully in a blazer.

Fact: a well-cut tailored jacket has a high, close armhole, which permits arm movement without dragging the whole jacket. A low armhole feels more comfortable standing still and restricts you more in motion, which is the reverse of what people expect in the fitting room.

Myth: alterations are for expensive jackets only.

Fact: taking in the sides of an inexpensive blazer costs little and does more for how it looks than spending twice as much on an unaltered one. It is the highest-return money in a wardrobe.

Myth: a tailor can fix anything.

Fact: a tailor can fix almost anything except the shoulder and, on many jackets, the armhole depth. Those two are why you shop for the shoulder first and everything else second.

FAQ: Blazer Shoulder Questions, Answered

How do I check the shoulder in a fitting room?

Stand naturally, arms down. The seam should sit at your shoulder bone with no dent below it. Then look at your back in a second mirror.

What if it fits the shoulder but is huge everywhere else?

That is the good problem. Sides, waist and sleeves are all routine alterations.

Can shoulders ever be narrowed?

Technically yes, and it is expensive enough that it is rarely worth it. Get a quote before assuming.

Should I size up or down when I am between sizes?

Up, if the shoulder is right in the larger size. Removing fabric is easy; adding it is limited by seam allowance.

Do oversized blazers still need a correct shoulder?

They need the shoulder the designer intended - usually a deliberate drop. Judge whether the drop looks designed or accidental.

How much should taking in the sides cost?

It is one of the cheaper alterations and one of the most effective. Ask for a quote on the waist and sides together.

TL;DR:

• The shoulder is the structural foundation - sleeve head, canvas, pad and armhole are all built around it.
• Buy for the shoulder and alter everything else; sides, sleeves and waist are routine work.
• Correct shoulder: seam at the bone, no divot below it, no diagonal pulling across the back.
• Oversized blazers are cut with a deliberate drop; a regular blazer sized up is not the same garment.
• Taking in the sides of an inexpensive blazer is the highest-return money in a wardrobe.

Everything else is negotiable. The shoulder is the jacket.