Though rarely used today, slant boards (also known as leaning boards) were commonly found on movie sets. They were designed to allow reclining without wrinkling neatly pressed costumes, reclining when the costume was too snug for the actor to sit or reclining when the immense weight of the costume limited mobility.
Here are some photos of slant boards being used on the set of 20th Century Fox films:
The Fox Archives is mandated to collect, catalog, preserve and make accessible the following assets of the 20th Century Fox studios: props, set decoration, photographs, art department and publicity materials from our film and television productions, and from the 20th Century Fox studio itself. We work primarily with internal Fox groups but also from time to time with outside organizations such as the Museum of Modern Art in New York, and Chicago's Museum of Science and Industry.