For a typical men’s single‑breasted suit jacket, you button the top (or middle) button and leave the bottom button undone when standing.

Basic rule in one line

  • Two‑button jacket: Button the top, never the bottom.
  • Three‑button jacket: Sometimes top, always middle, never bottom.
  • One‑button jacket: Button the single button when standing, unbutton when sitting.

Why the bottom stays undone

  • Modern jackets are cut so the bottom button is decorative and fastening it pulls the fabric and ruins the V‑shape.
  • Leaving it open lets the jacket drape cleanly over the hips and makes moving and sitting more comfortable.

Standing vs sitting

  • When you sit down in a single‑breasted jacket, unbutton it so the front doesn’t strain or crease.
  • Double‑breasted jackets usually stay buttoned even when sitting, often with the inner “anchor” button and main front buttons fastened.

Double‑breasted note

  • Double‑breasted: Fasten the inside anchor button and main working buttons; the very bottom front button is often left undone for ease and a cleaner line.
  • Unlike single‑breasted, these are designed to look right while fully or mostly buttoned almost all the time.

Information gathered from public forums or data available on the internet and portrayed here.