Flat-felled seam (jeans, double-topstitched)

The flat-felled seam in jeans construction (Kappnaht) is the classic double-topstitched garment seam of the jeans, workwear and outdoor industry. Two fabric pieces are first joined right sides or wrong sides together with a plain stitch. One seam allowance is then trimmed narrow (about 3 mm), the second (about 12 mm) folded around it, and the resulting 'fell' is secured by a second parallel topstitch in a single pass on a double-needle flat-felled machine.

Result: a flat, fully enclosed seam with no raw edges. Typical tensile strength 100-180 Newton per 50 mm (ISO 13935-2) - significantly higher than a standard lockstitch seam (60-90 N). Classic locations on jeans are the inseam, the front pocket bag seam and the yoke seam on the back leg. On workwear (dungarees, cargos, multi-function coveralls) the outside seam and crotch seam are also flat-felled.

The classic topstitch colour is ochre or orange thread (denim cotton or corespun with polyester core), giving the seam its typical look. Stitch density: 6-8 stitches per cm. For higher requirements (safety jackets to EN 342, EN 1149) aramid or FR threads are used. Key double-needle flat-felled machine makers are Union Special (US 35800), Juki (MH-380), Duerkopp Adler and Sunstar; sewing speeds up to 4,500 stitches per minute.