164 the user determines inside measure 164 by selecting the desired throat 166 to suit construction needs throat 166 may also be referenced as an inside radius of the elbow fitting the user locates the desired throat on first data table 30 and selects the corresponding inside measure 164 when the user cuts workpiece 50 along the first and second cutting planes three segments result shown in fig1 as first segment 160 second segment 170 and third segment 162 third segment 162 is in the shape of an isosceles trapezoid when a face on which miter cuts are made is viewed perpendicularly by rotating third segment 162 by 180 degrees about its longitudinal axis and joining the three segments at the cuttings planes the user can form a 90 degree three segment elbow 280 having two 45 degree miter joints and a desired throat 166 as shown in fig1 fig1 and 14 show a five segment 90 degree elbow fitting that can be formed by selecting a miter cut angle 152 of 11 25 degrees which may be accomplished by using 11 25 degree straight edge of layout tool 10 to lay out and cut the angled faces of workpiece 50 five segment 90 degree elbows are desirable since their use results in less resistance to air flow in a completed duct a five segment 90 degree elbow 380 can be formed by marking a second cutting plane 250 a third cutting plane 152 and a fourth cutting plane 252 on workpiece 50 using layout tool 10 the angle between first cutting plane 150 and second cutting plane 250 is bisected by a plane perpendicular to the longitudinal.