It is the most quietly reliable outfit in most wardrobes, and also the easiest one to get subtly wrong. A blazer with jeans can read as polished and slightly off-duty, the sort of thing that works for a school meeting, a lunch, a gallery, and dinner afterwards. Or it can read as the thing you grabbed because the real outfit was in the wash. The ingredients are identical. The difference is proportion.
Having tried most of the permutations on myself and on friends who called me in despair from their bedroom floors, I've come to think there are only three decisions that genuinely matter. Get those right and the rest is personal taste.
Start at the shoulder, not the length
A blazer lives or dies on its shoulder line. If the seam sits noticeably past the edge of your own shoulder, the jacket needs to be intentionally oversized everywhere else too, with a longer body and a roomier sleeve, or it just looks like it belongs to someone taller. If the seam sits exactly on the bone, you can get away with almost any length.
For most people, two shapes cover everything. The first is a soft, slightly roomy single-breasted jacket that hits at the top of the hip, sleeves pushed or rolled to show a wrist. The second is a longer, straighter jacket that finishes mid-thigh and skims rather than nips in. What tends not to work with casual denim is the very structured, very fitted, short interview blazer, which immediately asks for tailored trousers and makes jeans look like a compromise.
Fabric matters more than people expect in autumn. A blazer in flannel, wool, brushed cotton or a textured tweed looks expensive next to denim because the two fabrics have visual weight in common. A thin, shiny suiting blazer next to heavy indigo denim reads as two unrelated outfits sharing a body.
