It also feels funny that it took me so long to act on my devout love for fashion history, as I recall sketching robes à la française as far as the tender age of (around) 12; I believe my parents have kept some of those amateurish drawings somewhere, they seem to keep everything anyway.
The robe à la française (or sack dress, sack-back gown, saque, etc) is the typical 18th-century staple of women's fashion.
The shape of it remained mostly unchanged from about 1740 until the 1789 French Revolution, having evolved from the robe volante of earlier years (a sort of an unfitted and free-floating version of the sack gown), which in turn evolved from late 17th century/early 18th century's idea of comfortable daywear for women.
The front and sides are tightly fitted on the bodice, but the back flows from shoulders to the ground in a lavish curtain-like effect. Back in the early 18th century, the painter Watteau represented elegant women in such stylish gowns, though at the time they were mostly theatre costumes. His vision of post-Louis XIV elegance stuck however, Watteau is closely associated with the early rococo style of the Régence (1715-23) that followed the Sun King's death.
By the mid-1700s, the robe à la française could be a formal gown (ranking below strict court dress) or somewhat-dressed-up daywear (ranking above caracos and other types of shorter jackets) depending on the fabric and decoration. Highly-detailed embroidered silk satin or damask would make for stunning evening wear (with a decorated stomacher, very wide panniers and expensive lace oozing from the collar and cuffs), lightly embroidered or plain silk taffeta for casual daywear.
