When a guitar string is plucked, for example, a stationary wave is formed. This happens because on releasing the string, two progressive waves move down the string in opposite directions. When they reach the point where the string is tied down, they cannot move beyond this boundary. At the boundary, the wave is reflected and travels back in the opposite direction. The two reflected waves meet and interfere with one another. Superposition occurs, the opposing directions of the reflected waves cancel out resulting in one "stationary" wave.