The waistcoat, aka a vest, combined with a shirt and jacket completes the classic three-piece suit.
Did you know that waistcoats have their origin in Persia (modern day Iran)?
Waistcoats are one of the few clothing items that have a clearly marked beginning. King Charles II of England, in reaction to French fashion dominating the English court in the 17th century, decided in 1666 to adopt the Persian-style vest as the correct way of dressing in England.
"To Court, it being the first time his Majesty put himself solemnly into the Eastern fashion of vest changing doublet, stiff collar, bands and cloak, into a comely dress after the Persian mode, with girdles or straps, and shoestrings and garters into buckles...resolving never to alter it, and to leave the French mode"
The initial length was longer than the modern day waistcoat, and gradually reached today's standards in the 18th century.