Abstract: A device for folding flaps of box blanks having crease lines transverse to the travel direction of the blanks, for use in a folder-gluer. A rotary member is rotatable around a transverse axis and provided with at least one folding arm. A removable linking device linking to a support secured to a frame of the folder-gluer. An adjusting device adjusts the position of the folding arm with respect to the support. A motor drives the rotary member. A synchronization device of the motor comprises an encoder in desmodromic connection with a drive mechanism for driving the box blanks in the drive direction and a detector of the passage past a reference position of the box blanks on the folder-gluer.