When a taut string is plucked, the resulting wave vibrates up and down. This is produced by the interference of two waves traveling in opposite directions, having the same frequency and amplitude. Ideally, when a traveling wave meets a boundary, it gets reflected back and forth. The wave pattern remains stationary along the string while its amplitude fluctuates. Such a wave is called a standing wave. The sum of the individual wavefunctions of two waves gives the wavefunction of the standing wave. Here, the sine function represents the sinusoidal simple harmonic oscillation of the wave, while the cosine function acts as a scaling factor that modifies the wave's amplitude. When time equals an integral multiple of half a period, the two waves are in phase; they go out of phase for an integral multiple of one-fourth of a period. The points where the sine function is zero have zero displacement, and are called nodes. The points where the sine function is maximum, for in-phase waves, correspond to maximum displacement. These points are called antinodes.