Image illustrating how an oscillator (such as a guitar string) can vibrate at different frequencies called harmonics. The simplest and lowest frequency (in red at the bottom) is called the fundamental frequency. Still remaining fixed at either end, the string can oscillate with higher frequencies. These waves are called standing waves because they do not progress along. Instead they are produced by the interference of equal but opposite waves travelling along the string in opposite directions. The still points or nodes (orange balls) occur at points where the troughs and peaks cancel each other out.