Starrett 81573 Angle Plates 6 x 9 x 12 4F 4-Face Granite, Grade AA - Laboratory. Angle plates provide a convenient and practical means of clamping and holding work in a vertial position. Their excellent finish and flatness make them very compatible for use with granite surface plate accessories. The angle plates are available with either 2 or 4 finished faces. The 2-face angle plate has the bottom and the adjacent square face finished flat and square to one another. The 4-face is similar to the 2-face, but has the two adjacent sides finished flat and square to the other two faces, as well as being parallel to each other.