Abstract: Yarn is treated in an air entangling apparatus having a yarn passageway through which one or more multifilament yarns are fed and an air inlet duct intercepting the bottom of said passageway at right angles, the longitudinal axis of the inlet duct passing through or close to the longitudinal axis of the yarn passageway. A jet of air is introduced into said passageway and strikes the upper wall thereof forming a plural vortex turbulent zone in the upper portion of the yarn passageway which entangles the multifilament yarns. An air outlet duct intercepts the top of the yarn passageway and serves to create an air flow which holds the yarn in the upper portion of the passageway. Yarn is fed into the passageway with a small degree of overfeed, generally from about 0.1 to 10%. A structure, such as a rod, extends into the yarn passageway and contacts the yarn fed therethrough. The resistance to yarn movement resulting from this contact creates yarn slack in the area of the turbulent zone.