d of the effect yarn 16 is shown as a percentage on the ordinate of the coordinate system of fig2 the value 100 corresponds to the web thickness which is always the same in the embodiment the yarn length l of the effect yarn 16 is given in mm on the abscissa of the coordinate system the section represented by the course of the curve 30 of the effect yarn 16 which comprises the piecer 31 has a length of about one meter in fig2 beginning on the left in the course of the curve 30 the last effect 32 before the end of the effect yarn 16 which has been returned for piecing is shown the part of the web 33 following the effect 32 has been introduced as a yarn end into the spinning rotor 11 for piecing the effect 32 has an effect thickness of 150 of the web thickness the line 34 indicates the location at which the formation of the effect yarn 16 according to the specifications of the yarn repeat was interrupted the piecer 31 then follows and subsequently the web 35 the line 36 indicates the location at which the formation of the effect yarn 16 according to the specifications of the yarn.