In 1891, artist Charles Lasar patented this device to help draw angles accurately. The device consists of a wood frame with a screw eye (B) attached on the inside of the frame, halfway along one of the inside edges. A plumb bob (C) hangs from that screw eye, with the free end stretched across a series of graduated angle marks radiating from point B.
Lasar describes how to use it: "The frame is held up vertically, so as to show within its outlines the object to be drawn—in this case a house. The plumb-line is brought parallel with one corner of the house or the chimney and thus maintains the frame in a vertical position. Now look through the screw eye at the apex of the gable and bring the loose string to coincide with the left line of the roof. Holding the string against the frame, lay the frame over the drawing-paper and trace thereon the direction of this roof-line. Get the other gable line in the same way, then the ridge-line, and so on, as may be necessary, and the resulting drawing must be correct in perspective."