The three tailored jackets a man might own are frequently used as interchangeable words. They are separate garments with different origins, and the differences show in pockets, cloth and structure.
The suit jacket exists as half of a pair
A suit jacket is cut from the same cloth as its trousers, in the same dye lot, and the two are intended to be worn and to age together.
That relationship drives its details toward restraint, since anything visually assertive would compete with the matching trousers rather than complement them.
Worn alone, a suit jacket usually reads as an orphan, because the cloth is finer and flatter than anything designed to stand on its own beside a different pair of trousers.
The blazer descends from uniform
Blazers originate in club and naval dress, which is where the metal buttons, solid color and often navy cloth come from.
The uniform lineage explains why blazers accept patch or flapped pockets and heavier cloth without looking incorrect, since they were never half of a matched set.
A modern blazer is essentially a formalized odd jacket, sitting above a sport coat in formality and below a suit, and that middle position is the whole of its usefulness.
The sport coat came from country wear
Sport coats developed for shooting, riding and rural activity, which is why the traditional cloths are textured wools built for warmth and abrasion resistance.
Tweed, herringbone and checks belong to that lineage, and their visual activity is a functional inheritance rather than a decorative choice.
Details such as action backs, bellows pockets and elbow patches survive from the same origin, and they look deliberate on a sport coat and out of place on a suit.
Construction differs beneath the cloth
Suit jackets are usually built with more internal structure to hold a clean line, because a suit's effect depends on a uniform silhouette from shoulder to hem.
Sport coats are more often softly constructed, with lighter canvas and less shoulder padding, which lets a textured cloth drape rather than sit rigidly.
That difference is why a soft sport coat is comfortable over several hours and why a structured suit jacket photographs sharper while feeling more like equipment.
Where the distinctions actually matter
Pairing rests on contrast: an odd jacket needs trousers that differ clearly in color and texture, or the result reads as a failed suit.
Formality follows from cloth and buttons rather than from what the garment is called, so a smooth navy jacket with horn buttons behaves differently from the same shape in tweed.
Buying a suit intending to split it is the most common error, because the cloth that makes a good suit is the cloth least able to carry a jacket on its own.