Abstract: A device attached to a spinning or twisting machine, designed to monitor the integrity of two threads merging under tension into a yarn, has a carrier in the form of a disk or a stem rotatable about an axis and provided with a thread guide such as an eyelet, or a pair of pins parallel to that axis, bracketing the yarn downstream of the merger point. A rupture of either thread, detected by a separate sensor or by one of the bracketing pins, generates or releases a force which rotates the carrier around its axis by about half a turn to impede the advance of the remaining intact thread whereby the latter also breaks and the machine is stopped automatically, or manually in response to an alarm signal.