A set including a first panel, a second panel, a back panel, and a mechanical locking device for locking of a first edge surface of the first panel to a second edge surface of the second panel at a junction plane. The mechanical locking device includes at least one rod-shaped element at the first edge surface and at least one insertion groove at the second edge surface, the rod-shaped element is configured to be inserted into the insertion groove. The mechanical locking device includes a first panel groove on a first panel surface on the first panel and/or a second panel groove on the second panel surface of the second panel. The back panel is configured to cooperate with at least one of the panel grooves, and a locking gear configured to cooperate with the back panel and a panel groove, for locking of the first panel to the second panel.