Abstract: Apparatus and processes are described in which a friction twist imparter is used to twist two filaments or filament bundles while passing through a processing zone in which the filaments are textured by imparting a temporary high twist to the filaments and heat-setting the twisted filaments. Variations in twist density due to variations in the co-efficient of friction between the ply-yarn and twist imparter are controlled by separating the ply-yarn into two separate filaments or bundles of filaments at a point beyond the friction imparter and sensing variations in the position of the separation point, the wrap angle or tension, or both, of the ply-yarn leaving the friction twist imparter being adapted to such variations. The means for this particularly described are mechanical and include a spring-loaded lever projecting between the separated filaments at the separation point so as to follow changes in the position of this point.