The black nylon is designed to contrast with the material removed from the teeth,
THINNEST POH Floss
Description
Percept 420 The thinnest floss POH has ever produced. Made from six gathers of seventy filaments of tiny nylon, this floss slips between the tightest teeth. The black nylon is designed to contrast with the material removed from the teeth, to aid in learning proper flossing technique.