What is the difference between a 180x200 cm and a 180x210 cm mattress?

The 180x210 is 10 cm longer. For most couples of average height, a 180x200 mattress is sufficient. If you or your partner are 180 cm or taller, the extra length quickly becomes relevant, giving more room at the feet and allowing both sleepers to fully stretch without their feet hanging over the edge. The 180x210 format is also proportionally better suited to beds with a rectangular frame, since the slightly elongated shape sits more naturally in the room than a near-square format.