When measuring a left starter tread the throat of the tread is determined by the width of your staircase. The turn out depth is typically the deepest part of your starter tread, our standard measurement is 12.75 inches. The tread depth is the depth of the part of the stair tread you step on, our standard measurement is 11.5 inches. The length of the tread is determined by the requirements of your individual staircase such as the type of newell post and railing design. Remember the tread overhang when measuring your starter tread, typical overhang is 1.25 past the riser. For example, if your starter tread has a turnout depth of 12.75 inches you will need a starter riser that is 10.25 inches in turnout depth as there is a 1.25 inch overhang on the front and back.