The disclosure relates to a trapezoid ring, the production of a groove for the trapezoid ring in the piston, and an assembly, including a piston having a trapezoid ring of this kind. 
Trapezoid rings for pistons, which are inserted into corresponding grooves in the ring zone of the piston, are known, e.g. from EP 0 398 993 B1 (DE 689 19 074 T2), DE 41 19 908 A1, DE 11 2011 100 377 T5, DE 10 2007 057 840 A1, DE 196 16 474 A1, and DE 30 345 19 C2 or DE 944 761.  However, the trapezoid annular grooves disclosed in the abovementioned documents are not suitable for receiving trapezoid rings having a small axial overall height, and this leads to a relatively large extent of the piston in a direction parallel to the piston stroke axis. 
DE 43 41 386 C2 discloses a seal unit for pistons of internal combustion engines having an annular groove arranged in the fire land of the piston, the annular groove having an upper groove flank of conical design for receiving a piston ring designed with a corresponding flank.  Here, the piston ring rests on the upper region of the piston, that facing the combustion chamber, leading to increased wear of the piston ring. 
Owing to the overall height hitherto used for trapezoid rings, it is problematic, especially under series production conditions, to produce a trapezoid groove which decreases in axial height from the outside in the direction of the piston stroke axis.  On the other hand, however, there is the requirement for a small height of trapezoid rings in order in that way to reduce the piston height.