SINGER 14T967DC-WORKBOOK QUANTUMLOCK - Manual (Page 91)

Three-Thread Flatlock (Wide)
Three-Thread Flatlock (wide) page 1
The Three-Thread Flatlock (wide) has the same set up as the Three-Thread
Flatlock (narrow) Stitch. The only exception is the Right Overedge Needle is now
unthreaded and removed and the Left Overedge Needle is threaded. By using the
left needle, the stitch becomes wider.
Right Overedge Needle Thread
Left Overedge Needle Thread