An instrument for stretching the skin comprising an elongate, dimensionally stable guide with at last two modules that are disposed on said guide and comprise anchoring means for securing to the skin, wherein at least one of said modules can be moved on the guide in the longitudinal direction thereof, and a non-return device allows movement thereof in a movement direction and blocks movement thereof in the opposite, reverse direction, said non-return device being able to be inverted in terms of the movement direction and the reverse direction.
