Although it was developed from the daybed in the 17th century, it wasn't until the 18th century that the chaise longue first attained great popularity. Like anything, it spawned variant forms, including the Duchesse brisee, an upholstered chaise longue consisting of either a round-backed armchair with a matching but separate footstool, or two of these chairs facing each other with the footstool sandwiched between them. the separate elements (three) could be attached to one another, but each could also be used on its own. The duchesse brisee was also popular in England, where it was known simply as a duchess. As in: "Don't bother me for the next few hours, for I'll be dozing on the duchess."
Another variation was the Duchesse en bateau: an 18th-century French chaise longue with an upholstered foot enclosed by a low, curving wall of wood, which matched the curve of its back.
Along Comes the Lounger
Not surprisingly, somewhere along the line, the unfamiliar-looking spelling was misinterpreted as chaise lounge. This is considered an error in British English, but is a common and accepted variant in United States English. It was further perpetuated by the above-mentioned term, to lounge, or lounging. For the past several decades, garden furniture catalogs (or is it catalogues?) have pictured chaise lounges, proving that the term has been around for a number of years and is likely here to stay. With apologies to the French.
Chaise longue: /SHāz ˈlôNG/