Detail of Concert Champêtre, Jean Baptiste Joseph Pater, ca. 1734; MMA 37.27
[Edit, 1/14/2013: Newcomers, you may want to see my tag for the sacque, which will show you all of the Galerie des Modes française/sacque plates I've translated so far. Paintings are great, but labeled fashion plates are even better.]
The robe volante was unfitted and loose, with pleats in front of and behind the shoulders that hid the wearer's shape. The front generally met or crossed-over without a stomacher, and when the gown was worn with a wide hoop it provided a good setting for textiles with a large pattern woven into them.
The Declaration of Love, Jean-François de Troy, ca. 1731; Charlottenberg Palace
As the robe volante became worn more often, it also became more fitted - but only in the front. The addition of a more formal front with robings and a stomacher, and an open skirt that revealed a decorative petticoat, made it more acceptable to move out of the sphere of negligée clothing. At first, sleeves were worn cuffed, as in the mantua, but once the française developed as its own style (ca. 1735), it was much more common for sleeves to be decorated with ruffles of self-fabric, which were then filled in with delicate engageant ruffles sewn to the sleeve of the gown or shift.
Madame de Pompadour, François Boucher, 1756; Alte Pinakothek
The sacque was informal compared to the French court gown, but it was worn by the highest members of society in all situations that did not call for court dress. By that point, it had been exported from France and could also be found in Britain and America. A negligée type of the style still existed in the pet-en-l'air, a shorter version similar to a jacket that would only be worn in the home.
Detail of Tête à Tête, part of Marriage à la Mode, William Hogarth, 1743-1745; National Gallery NG114
The robe à la française fell out of favor as Marie Antoinette's court began to prefer simpler styles to fit their idealized idea of country life. Where France went, the rest of the fashionable word followed, and by the end of the 1770s the sacque was seen no more.