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''Guitars, like other string instruments, primarily generate sound through the '''resonance''' of their strings.'' The strings have elasticity (ie they can bend) and have tension applied to them so that they are suspended in a straight line - their '''resting''' state. Any force that moves the string outside its resting state increases the amount of tension on the string - this is called '''exciting''' the string. It will try to return to its resting state; however, its momentum will actually carry it past that point, like a pendulum. Thus, the string resonates back and forth, slowly losing energy by transferring it to the body (as well as generating heat inside the string).

''Strings resonate at a certain pitch, depending on the thickness, length, and the amount of tension applied to the string.'' The thicker and longer the string is and the lower the tension on it, the lower its resonant pitch is, and vice versa. It is important to realize that this simple formula isn't absolute - there are some very nuanced complexities to the physics involved; however, the relationships are good enough to be perceived as completely accurate. For guitar and most instruments, the strings have different thicknesses but relatively the same tension. Thus, the thicker strings have lower pitches than the thinner ones. The even tension makes playing (and especially bending) the strings have a consistent difficulty from one to the next. The primary method of altering pitch is by shortening the string length by pressing the strings against the frets or fingerboard. However, other methods are also used - for instance, in electric guitar a whammy bar or bending technique may be used to reduce or increase string tension. In contrast, the piano matches each of its keys to an independent string of fixed length.