The introduction of vest

A waistcoat has a full vertical opening in the front, which fastens with buttons or snaps. Both single-breasted and double-breasted waistcoats exist, regardless of the formality of dress, but single-breasted ones are more common.

Before wristwatches became popular, gentlemen kept their pocket watches in the front waistcoat pocket, with the watch on a watch chain threaded through a buttonhole. Sometimes an extra hole was made in line with the buttonholes for this use. 

A custom still sometimes practised is to leave the bottom button undone. This is said to have been started by King Edward VII (then the Prince of Wales), whose expanding waistline required it. Variations on this include that he forgot to fasten the lower button when dressing and this was copied.

When it is fastened, the bottom of the waistcoat pulls sideways causing wrinkling and bulging, since modern waistcoats are cut lower than old ones. This convention only applies to single-breasted day waistcoats and not double breasted, evening, straight-hem or livery waistcoats that are all fully buttoned.