A very useful feature of all Union Special butt seam styles is their ability to sew a Single–Ply Trimming Lap Seam.  Each of these styles comes with an edge guide (part number 36203 and screw 22849) that can be easily attached to the left or right side of the throat plate in front of the presser foot.  When this edge guide is attached to the left side of the throat plate, Figure 3, the right ply of material is then butted to the inside edge of the guide and the left ply of material is laid over the top of the guide.  As the material is fed into the presser foot and knife system, only the left ply is trimmed.  As the sewing continues the left ply will overlap the right ply to give a smooth lap finish in top.  (However, the bottom [right ply] will not be trimmed, so depending upon how well the material is guided the finish may be inside or outside of the bottom coverstitch.)  Configuring the machine with an edge guide, therefore, gives a Lap Seam finish with a Butt Seam machine.  In this case the left ply on is on top and right ply on the bottom, Figure 4. If the edge guide is attached to the right side of the throat plate, Figure 5, the left ply gets butted to the edge guide and the right ply is trimmed.  This results in a Lap Seam finish with the right ply on top and the left ply on the bottom, Figure 6.