Here are some modern flags, held in position by a C-stand. These allow a director of photography in a photo lab or a movie shoot to control exactly which surfaces the light touches or to keep a light from shining into the lens.
Sargent placed a chair behind the sitters, with poles strapped to the back of the chair. Across those poles he draped a piece of cloth as a cutter to reduce the backlighting and the glare into his eyes. The soft key light comes from a window unseen to the right.
New book on Sargent (released yesterday):John Singer Sargent: Figures and Landscapes, 1900-1907: The Complete Paintings, Volume VII
More about that Sargent painting.
Sargent photo from "Representational Painting" Facebook page
Photo of modern cutter from Enchanting Kerala.org
Previously on GJ: C Stand