The starrett prosite protractor takes the error prone calculations out of the process of mitre cuts simply set your saw to the angle that you read upside down or backwards the reading is always correct this easy to use tool has two scales the red arrow and the red scale indicate the exact mitre saw setting for an accurate mitre joint every time the black arrow and the black scale indicate the exact mitre saw setting for fitting a single work piece to an angle it is ideal for carpenters plumbers