Description: This is a silk open robe known as a sack-back or sacque – because of the loose pleats that flow from the shoulders. Although this style grew out of informal wear, by the 1760s the sack-back – or robe à la Française – was the style of choice for formal occasions. This extravagant example, in saffron yellow silk woven through with metal thread, is French fashion at its most opulent.