A flat seam (or flatseam) is a 4 needle, 6 thread sewing machine, stitch class 607.
This is one of the more unique industrial sewing machines because the operator feeds the fabric face up and the presser foot has an oscillating knife to trim the fabric before it is sewn.
After trimming the fabric is lapped over before the 4 needles sew it together. The seam is very similar to a top and bottom cover stitch but it has 4 needles.
This stitch is commonly referred to as a "Flat lock" which was a trade marked name by the Willcox and Gibbs company but was a 4 needle/9 thread machine. A flat lock and a flat seam are two totally different stitches.
