Three words, three garments, and almost nobody uses them correctly. The difference is in the cloth and the construction, not the color.
Walk into any store and you will find the same jacket labeled a blazer on one rack and a sports coat on another. The words have collapsed into each other, which is a shame, because the distinction is genuinely useful when you are deciding what to buy.
Here is what each one actually is.
The Suit Jacket
The easiest to define. A suit jacket is cut from the same cloth as a matching trouser or skirt, and it is made to be worn with it.
Because it is half of a set, it is usually the most structured of the three: a fuller canvas through the chest, a more defined shoulder, a cleaner line. It is also the least versatile on its own, because a suit cloth worn alone tends to look like it has lost its other half. Fine, smooth suiting in particular gives the game away immediately.
The Blazer
Historically a solid-colored jacket in a plain cloth, most traditionally navy, with metal buttons and a slightly more relaxed construction than a suit jacket. The naval origin is where the buttons come from.
In modern use the metal buttons have largely gone and the word has widened to mean any tailored jacket in a plain color that is not part of a suit. That is a reasonable working definition and it is how the word will be used in any store you walk into.
A blazer is the most useful of the three to own. It is structured enough to raise the register of whatever is underneath and plain enough to go with everything.
The Sports Coat
The most casual and the most interesting. A sports coat is cut from a patterned or textured cloth: a tweed, a herringbone, a check, a hopsack. It was never part of a suit and never intended to be.
Construction is softer than a blazer's, with less padding and a more natural shoulder, which is why a good one is far more comfortable to wear all day.
The texture is the point. It is also enormously practical, because a patterned cloth disguises creasing and ordinary wear in a way a plain one does not.
Why the Distinction Is Worth Keeping
Because it tells you what a jacket will and will not do.
A suit jacket worn alone is the most likely of the three to look wrong. A blazer goes with almost anything. A sports coat is the most casual and the most forgiving, and the only one you would happily wear with jeans.
The cloth tells you which one you are holding. Plain is a blazer, patterned is a sports coat, matched is a suit.
How to Tell Them Apart on a Rack
Look at the cloth first. Plain and smooth suggests a suit jacket or a blazer. Textured or patterned suggests a sports coat.
Look for a matching bottom half on the same rack. If there is one, it is a suit jacket whatever the label says.
Look at the buttons. Metal, horn or leather buttons suggest blazer or sports coat. Buttons in the same cloth as the jacket suggest a suit.
Feel the shoulder. Firm padding with a defined edge suggests a suit jacket. A softer, rounder shoulder suggests a sports coat.
None of these is absolute, and taken together they are reliable.
Which One to Buy
If you are buying one jacket, buy a blazer in a plain mid or dark color. It goes over a dress, with a tailored trouser, with a skirt and with denim, and it raises all four. Nothing else in a wardrobe does that.
If you are buying a second, buy a sports coat in a texture. It covers everything the blazer does not: the weekend, the long lunch, the day that is smart but not formal. A textured jacket also survives being worn hard in a way a plain one does not.
Buy a suit only if your work requires one. Two separate jackets give more outfits than a suit, and they wear evenly.
The Fit Points That Matter
The shoulder seam must sit at the edge of your shoulder. This is the only fit point that cannot be altered economically, so check it first and everything else second.
The sleeve should end at the wrist bone, or a little above if you want a cuff to show. This is the cheapest alteration and the one that most transforms how a jacket reads.
The button stance, meaning where the fastening button sits, decides the shape of the opening. A higher stance reads more formal and a lower one reads longer. Fasten it in the fitting room and look at the shape the lapels make.
The waist is the second easiest alteration. Taking in the back seam by an inch turns a boxy jacket into a shaped one for very little money.
Wearing Each One
A blazer wants something simple underneath, because the jacket is the structure and the thing under it does not need to compete. A fine knit, a plain blouse, a dress with a clean neckline.
A sports coat wants the opposite: it is already textured, so keep everything else plain and let the cloth be the interest.
A suit jacket worn alone needs help. Put it with a texture or a color that clearly does not match, so it reads as a deliberate separate rather than an orphan.
The History, Briefly
The blazer comes from rowing and from the navy, which is where the metal buttons and the original navy cloth both come from. It was a club garment before it was a wardrobe one, and the brass buttons were a badge.
The sports coat comes from country sport, which is why the cloths are tweed, herringbone and check: patterns that were designed to disappear against a landscape and survive weather.
The suit came from neither, and arrived as a business uniform in the nineteenth century.
The history is worth a paragraph because it explains the formality order that still holds. The suit is the most formal, the blazer sits below it, and the sports coat sits below that. Nothing about that has changed in a hundred years, whatever the labels in the stores now say.
What to Wear Each One With
A suit jacket worn alone wants a clear contrast underneath so it reads as a deliberate separate. A textured trouser, a denim, or a skirt in a different cloth entirely.
A blazer is the most flexible. Over a dress, with tailored trousers, with a skirt, with jeans. The plainer the jacket, the more it will do.
A sports coat wants plain everything else. The cloth is the interest and anything patterned beside it competes.
One Practical Note on Buttons
On a two-button jacket, fasten the top one and leave the lower open. On a three-button, fasten the middle. On a one-button, fasten it or do not, but not halfway through an evening. Unfasten before sitting down, on all of them, which is what stops the front pulling into a diagonal.
Tailoring Worth Understanding
The Short Version
A suit jacket matches a bottom half. A blazer is plain and unmatched. A sports coat is textured or patterned and softer in construction. Buy the blazer first, the sports coat second, and the suit only if you have to. Check the shoulder before anything else, and pay to have the sleeve shortened.