The original trumeau mirrors were set into wood paneling, or boiserie (bwah-zer-EE), the fancy carved wall-covering of choice in the 17th and 18th centuries (images 2-4). As we've seen, glass was an expensive resource then, so at first it was unusual to have even small mirrors set into the decor. As mirror glass became easier to produce in larger squares, it would occasionally be incorporated into paneling.
In French, trumeau is the word for the thin section of wall between two doors or windows. The word was first used to describe a mirror on that section of wall in the early 1700s. By the middle of the century, it was used to describe a mirror above a mantle (in English, we call that a pier glass).
The trumeau mirror gained popularity among the growing upper-middle class, who were looking to emulate the wall-paneled aristocracy. One of the results of the Industrial Revolutions was the rise of the merchant class, who were making more money and then spending it on making their homes more like those of the ossified upper class in their old family estates with lots of wood paneling. In fact, some of these old estates were dismantled, with furnishings — and paneling — sold on the secondary market (image 5). So paneling was in. And mirrors were also in: by the middle of the 19th century, manufacturers had figured out how to make plate glass in large panels, so it was more practical and much less expensive to install mirrors. And so mirrors made out of old paneling, or with frames made to look like old paneling, would have also been in.
Most trumeau mirrors I've seen are Neoclassical or Empire in style: symmetrical, with sober rectangular shapes and classically-inspired motifs like acanthus leaves, scrolls and garlands, and ribbons. This was the style in the late-18th and early-19th century, and remained somewhat popular for the next century or so.
Why is the trumeau mirror back in vogue these days? I think it has something to do with the idea of salvage. People love the idea of finding old mouldings and paneling and upcycling or re-using it in a new context, right? And even reproductions bear an echo of the handcarved panels of yore. I'm not sure it'll be my choice for a pier glass, but I do like the idea of softening my white-box rental with some vestige of the past.
Images: 1 Restoration Hardware; 2 Boiserie from the Hôtel de Varengeville, Paris, c. 1736-52, at the Metropolitan Museum of Art; 3 Boiserie from the Palais Paar, Vienna, c. 1765-72, also at the Met; 4 Bibliothèque de l'Arsenal, Paris, c. 1745-50, via La Tribute de l'Art; 5 Pair of Louis XV trumeau mirrors sold in 2007 at Christie's for about $25,000; 6 A trumeau mirror in the petit bureau of the Nissim de Camondo house museum in Paris, the home of wealthy collectors in the late-19th and early-20th century (and probably my favorite small museum in Paris!); 7 Wisteria.