Later on, in the 1950s, Citroën developed a remarkable type of suspension. A high-pressure hydraulic system was used in over 9 million Citroën cars. They included the DS, SM, GS, CX, BX, XM, Xantia, C5, and C6. Self-levelling is the principal benefit – the car kept a constant ride height above the road. It adjusted to the passenger and cargo load, and gave a very soft suspension. This type of suspension smooths out road irregularities without disturbing the occupants. It is often compared to riding on a 'magic carpet' for this reason.