Gilet

A waistcoat is a sleeveless, close-fitting garment worn over the shirt and under the jacket, forming the third piece of a three-piece suit or an independent sartorial element. It provides an additional layer of warmth, covers the shirt and trouser waistband, and adds formality and visual structure to an outfit.

The bottom button of a waistcoat is traditionally left undone. This is not an oversight — it is a convention attributed to King Edward VII, adopted so widely that buttoning the last button now reads as an error.