St. Denis and Pierson supposed the sea as being a superposition of many simple regular waves, each regular wave with its own frequency (or length), amplitude and direction of propagation. The ship is supposed to respond on each of these individual waves as being a linear system. This holds that doubling the wave amplitude will result in a doubled response amplitude, while the phase lag between the response and the regular wave has not changed. A summation of the responses on each of these individual regular waves provides the irregular response in a seaway.