Abstract: A rocker mechanism (10) preferably intended for rotatable arm chairs, comprising an upper part (12) which is attached to a seat and which is pivotally journalled to an underpart (14) by means of a horizontal pivot shaft (24), the underpart being carried by the vertical supporting pin of, for example, a rotatable arm chair. The underpart presents an upstanding holder (36) having a throughpassing opening (38) for accommodating a locking rod (48) provided with a handle and extending parallel with the pivot shaft (24), and promontories (40) which have a trapezium-shape cross section and which are intended for abutment with recesses (41) in two clamping plates or jaws (76) located on a respective side of the holder. The locking rod (48) is journalled in bearing blocks (58) on the upper part (12) and presents a lefthand screwthread (50) and a righthand screwthread (52) which co-act with respective screwthreaded holes (51 and 53) provided in the clamping plates or jaws (76).