Flags and Cutters

A "flag" —also known as a "cutter" — is a piece of opaque black cloth or board held in position to block a source of light.

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. 

Here's a photo of John Singer Sargent painting Mrs. Fiske Warren (Gretchen Osgood) and her daughter Rachel at the Fenway Court in Boston (now the Isabella Stewart Gardner Museum), where Sargent had set up a temporary studio.

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.
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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