When to button a suit jacket depends on its front design and how you are wearing it. Common dress conventions are useful, but the garment should also close naturally without distorting its shape.
Identify the front style
For a conventional single-breasted two-button jacket, the upper button is generally the working button and the lower button is usually left open. A one-button jacket uses its single front button. Three-button designs and roll constructions need attention to where the lapel naturally turns.
Double-breasted jackets have a different fastening arrangement. Check the interior anchor and the intended front closure. Do not apply a single-breasted rule to every button visible on the front.
Use comfort as a fitting signal
A conventional single-breasted jacket is often unbuttoned when sitting so it does not pull across the waist. Button it when standing if that suits the setting and the garment. Do not force a closure that creates strong tension lines.
If the jacket feels comfortable open but tight when fastened, assess the fit. Leaving it open at all times may hide a sizing problem rather than solve it. Try the jacket with the normal shirt and any waistcoat before deciding.
What if you are wearing a waistcoat?
Check the entire outfit. The waistcoat should not create bulk that pushes the jacket away from the chest. Follow the intended dress convention for the event and the jacket's design. A fitting rehearsal makes these choices easier than adjusting buttons for the first time at the venue.
Related reading
Wearing a polo shirt with a blazer. For another useful reference, see How to document jacket sample approval.
