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. In a three piece suit, the cloth used matches the jacket and trousers.
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 bar on the end of the chain held the chain in place to catch it if it were dropped or pulled. Now waistcoats are worn less, so the pocket watch may be more likely be stored in a trouser pocket.
Wearing a belt with a waistcoat (and indeed any suit) is not traditionally correct. The waistcoat instead covers a pair of braces (suspenders in the U.S.) underneath it, to give a more comfortable hang to the trousers.
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. It has also been suggested that the practice originated to prevent the waistcoat riding up when on horseback. Undoing the bottom button avoids stress to the bottom button when sitting down; when it is fastened, the bottom of the waistcoat pulls sideways causing wrinkling and bulging, since modern waistcoats are cut lower than old ones.