If two organ pipes, for example, are tuned a few vibrations apart, they produce a throbbing tone when sounded together. If the difference is three vibrations per second, the waves will be out of phase three times in each second and will be in phase an equal number of times. When they are out of phase, there is a moment of comparative silence. When in phase, however, they reinforce each other and increase the intensity of the tone. Each produces tones in a different way.
Strings are perhaps simplest to understand.
The greater the tension on a string, the higher its pitch. A violin string, practically speaking, is under constant tension. The violinist raises its pitch by shortening the vibrating length of it with the fingers of the left hand. When a string is bowed, plucked, or struck near one end, it may vibrate in several ways at once. It vibrates as a whole, sounding the fundamental tone.