Describe how a stationary wave is formed at a boundary?

A stationary wave is a wave formed by the superposition of two progressive waves. These waves must have equal amplitude frequency and speed. They must also be travelling in opposite directions. When a wave is reflected off a denser medium at a boundary, it undergoes a phase change of pi radians. The reflected wave is then able to superimpose onto the incident wave. When the reflected wave and incident wave superimpose they form a stationary wave since they have the same amplitude, frequency and speed. Moreover they are also travelling in opposite directions.