The waistcoat ★

The bottom button

The bottom button of a single-breasted waistcoat is left undone. The convention is usually attributed to Edward VII, who is said to have left his open because he had grown too large to fasten it, a story that is repeated everywhere and documented nowhere.

There is a better structural explanation available: waistcoats are cut with a point at the front, and leaving the last button open lets the garment sit properly when the wearer sits down. Whichever is true, the convention is universal, and most waistcoats are now cut assuming it.

The odd waistcoat

A waistcoat in a different cloth from the suit (tweed with flannel, linen with cotton) is a legitimate and old idea, and it works when it reads as deliberate. The failure mode is a waistcoat that nearly matches, which looks like a three-piece suit that has gone wrong.

Backless waistcoats, with the back in lining and an adjuster, are a formal-wear device to reduce bulk under a tailcoat. In an ordinary suit they are a cost saving and they look it when the jacket comes off.