The pitch that a vibrating string produces is determined by three things: the diameter of the string, the length of the string and the tension on the string. Thick strings have lower pitch than thin strings. Long strings have lower pitch than short strings. Loose strings have lower pitch than tightly stretched strings. Guitars exploit all three parameters to control the pitch of the notes that are sounded.
String Size and Pitch
The standard guitar has six strings -- all of them a different size. The strings are numbered from thinnest to thickest. When the guitar is in playing position the thickest string -- the 6th string is on top and the thinnest string -- the first string -- is nearest the floor. Each string should be a few notes above the next lower string, if there is one, and a few notes below the next higher string, if there is one. Size is what makes the six strings different from each other. The strings are all the same length and are all under the same tension.
Tension and Pitch
The tension of each string is controlled by the machine heads -- a gear and turning knob that can be used to adjust the pitch. On some guitars, like flamenco guitars, the mechanisms that control the tension are wooden pegs that fit into holes in the guitars. Whatever the mechanism, it takes some time for the strings to settle into the guitar, and the guitar must be tuned every time it is used. The thin strings especially need frequent tuning. If you ever watch a guitarist change strings, you will see him grab the string near the middle and pull it to settle the string into place.
Length and Pitch
Length is what controls the pitch of a string during play. Once the correct tension is applied, the pitch of a string is controlled by pressing the string into the fretboard with one of the fingers of the left hand. There are implanted bars -- called frets -- running across the fingerboard, and when the string is pressed down its length is determined by which fret is chosen. Each fret is equivalent to the difference between adjacent keys on the piano. This is the smallest pitch difference recognized in Western music -- it is called a half step.
When a guitar string vibrates, it continuously changes its mode of vibrating. Sometimes it vibrates as a whole string with the largest movement in the middle of the string. Sometimes it vibrates as if it were two strings of half the length and the middle of the string hardly moves at all. It can also vibrate in three or more pieces. If you lightly touch a string at certain points, you can force the vibrating string to emphasize certain modes of vibrating. This technique does not change the length of the vibrating string, but it does force it to vibrate in several pieces, which changes the pitch and produces a bell like tone.
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