A blazer is the one coat-weight piece that quietly does the work of an outfit. The same jacket can look sharp, relaxed or borrowed from someone else, and the difference is rarely the price. It comes down to three measurements: where the shoulder seam lands, where the hem stops, and where the sleeve ends. Get those three right and a modest blazer looks considered. Get them wrong and an expensive one looks off.
The shoulder seam is the line where the sleeve joins the body of the jacket. On a classic fit it sits right at the edge of your shoulder, where the shoulder bone ends and the arm begins. If the seam sits inside that edge, the jacket pulls and the sleeve creases. If it hangs past the edge, the jacket has a dropped shoulder: the whole shape reads softer and bigger.
Neither is wrong, but they say different things. A seam at the edge looks structured and precise. A dropped seam looks deliberately relaxed, which is the idea behind most oversized blazers. The important part is that it looks chosen. A small, accidental drop of a centimetre or two usually just looks like a jacket that is slightly too big.
The shoulder is also the part to check first in a shop, because it is the hardest part of a jacket to change. A tailor can shorten sleeves and take in a waist, but rebuilding a shoulder is a major job that most people never pay for.
A classic blazer ends around the seat, roughly where your hand rests if you let your arm hang and curl your fingers. That is a starting check, not a law. Shorter hems cut the body of the outfit and leave more trouser showing, so the line from waist to floor looks longer. Longer hems turn the jacket into a layer that carries the outfit, which suits a slim or straight leg underneath.
The useful question is what the jacket sits next to. A cropped jacket meets a high waist. A long jacket meets a narrow trouser. If both are big at once, the outfit loses its shape. The slider above shows it: the further the hem drops, the less trouser is left to balance it.
The sleeve should end around the wrist bone, so that a centimetre or so of shirt cuff shows. Too long and the sleeve swallows your hand; too short and the shirt disappears. Sleeves are also the easiest fix. A tailor can shorten them at the cuff in most cases, so a jacket whose shoulder is right and sleeve is a little long is a good buy.
Oversized blazers break this rule on purpose: the sleeve can run to the base of the thumb, pushed up once if it is too long. That only works when the shoulder is also clearly dropped. A long sleeve on a seam that sits at the edge looks like a mistake.
Close the top button and look at the front. The fabric should lie flat. If it pulls into an X shape around the button, the jacket is too small in that spot; if it ripples, it has more room than the cut intended. Neither needs a verdict on whoever is wearing it. It is a signal about the garment. Many people leave a blazer open anyway, and that is fine as long as it still looks right closed.
Shoulder first, hem second, sleeve last: the order of what is hard to change.
Every rule above has a version where ignoring it is the point. Dropped shoulders, long sleeves and a hem past the seat are the backbone of oversized tailoring, and cropped boxy jackets turn the hem rule upside down. What stays constant is that the choice is visible. Break one rule firmly rather than three half-heartedly, and keep the rest of the outfit simple around it.