An interlocking device arranged to prevent forbidden order of engagement/disengagement of two tooth clutches in a vehicle transmission is provided and includes a groove arranged in a first of shift rods, and a second and a third groove arranged in a second of shift rods, an interlocking element arranged coplanar with the shift rods, and directed and movable towards the shift rods. The length of the interlocking element is adapted to allow certain combinations and order of engagement/disengagement of the tooth clutches. A piston element partitions the second and third groove. The piston element is movably arranged in a hole in the second shift rod such that the interlocking element can push the piston element down in the hole, allowing the second shift rod to be displaced only one-way when the first shift rod is in an engaged position.