Disclosed is a fixture for the reed of a seam weaving machine used for joining two opposite ends of a synthetic fabric by means of a woven seam. The reed is provided with pivotally mounted combs (22) for sliding seam weft threads to the selvage. The fixture comprises an inclined rail (24) and a pressure rail (30) which are disposed at a distance from the bearing mechanism (20) and apply opposite torques to the combs (22), as well as devices (26, 32) for positioning the inclined rail (24) and the pressure rail (30) at angles relative to the bearing mechanism (20) in order to stagger the combs (22) in the position thereof by means of the inclined rail (24) and the pressure rail (30) so as to operate the machine in a first mode of operation. A roll is also provided which can be displaced along a track across the width of the reed in order to successively swivel the combs (22) towards the selvage so as to operate the machine in a second mode of operation. The inclined rail (24) or the pressure rail (30) can be removed from the combs (22) to operate the machine in the second mode of operation. A U-shaped rail (40) can be placed on the front side of the combs (22) in the second mode of operation, the bottom leg (42) of said U-shaped rail (40) impinging the combs (22) below the shaft (20) and the top leg (44) thereof impinging the combs (22) above the shaft (20).