Open, semi-circular protractor. Large, but very thin. Base bar has rectangular aperture with sloped edge and mark for center of angle measurement. On left side of base bar is another rectangular aperture slightly skewed from the diameter line. The edge of this aperture is marked as a minute index, 0-60 by 5. After drawing or placing the protractor along the line and finding its angle to the nearest degree, the protractor is flipped on the line and re-aligned with the line to be measured going along the diameter line. The line will cut through the minute aperture and intersect with the minute scale. This works similar to a protractor with an index arm, where the two protractor scales are used as a diagonal scale with a minute scale on the index arm (see M-110).
