passage 81 is easier to machine than yarn passage 74 in the jet device 90 of fig9 there is provided a water exit channel 91 separate from the yarn passage 55 to facilitate water removal referring now to fig1 and 11 there is shown a liquid jet device 100 in the form of a cylindrical housing 101 having an insert 102 in which there is a bore 55 defining an axial path for the yarn 13 to pass through the jet 100 the bore 55 may be conical as bore 74 of fig7 or stepped as bore 81 of fig8 water or other suitable liquid is supplied in the direction of arrow a to the annular space 106 between the housing 101 and the insert 102 in the insert 102 are water channels 108 which are tangential to the bore 55 two such water channels 108 being shown in this case symmetrically disposed around the yarn 13 the water channels 108 being offset from the axis of the bore 55 provide that the impinging jets of water subject the yarn 13 to a torque which false twists the yarn 13 the water channels 108 which may be straight as shown or may be formed spirally in the insert 102 are directed at an angle to the direction of running of the yarn 13 so that the water jets have components of velocity along the path of the yarn 13 as well as laterally thereof this applies a forwarding force to the yarn 13 as well as the false twisting torque the greater the cone angle c the more is the twisting torque and the less is the forwarding force and vice versa the water may exit from the insert 102.