This set consists of a steel rule, a square head, a center, and a protractor head. You will need them all for general measurements or initial layout. The 12-inch steel rule is calibrated to a 32nd of an inch and mm. The square head will measure 60° to 45° angles, or lines parallel to an edge. The protractor head measures to 1° accuracy. Both the square and protractor heads are equipped with bubble levels. The center head is used for locating the center of any round workpiece.