In the first example above the lines are aligned at an angle running down. The resulting cliff face is more resistant to erosion as the waves are working against the lines of weakness. In the second example the lines are aligned at an angle running up. The resulting cliff face is easily eroded as the waves are able to attack the lines of weakness. The cliff is easily undermined forming caves and arches.
Height is the distance from the top of the wave to the bottom of the trough between two waves.
Length is the distance from the top of one wave to the top of the next wave. The distance between waves is the most important as it determines the frequency of waves. It is the frequency of waves that result in waves being either constructive or destructive. When a wave reaches shallow water its top falls forward. When a wave breaks at the rate of ten or less per minute each wave is not affected by the following wave, so the swash is more powerful than the backswash.