The Structure of the Acoustic Guitar
Six strings, each with a higher pitch
How to refer to the six strings
Guitars typically have six strings. Each string has a different thickness. Starting from the thinnest string, the strings are called string 1, string 2, and so on, up until string 6. Strings 1 and 2 are called "plain strings" and are bare steel strings (unwound). Strings 3 through 6 are wound with metal. When holding a guitar, string 6 is the topmost string.
The thicker the string, the lower the pitch
Moving from up to down (i.e. from thicker to thinner) result in an increasingly higher pitch. The diagram below shows ordinary tuning, which refers to the tone produced from each string when not held down with the left hand.

The pitch becomes lower as you move up the strings

Sound of open guitar strings
Sound of open strings, from string 1 through string 6
Moving one fret increases the pitch by one semitone
As strings become shorter their pitch increases. Guitars are designed to use this property so that the pitch they produce increases a semitone each time the position the string is held down at changes.
The metallic parts on the neck are called frets. A player uses his or her left hand to hold the strings down in the spaces between the frets. There are a total of 20 frets for 20 semitones. This means that, for instance, string 6 can play from low E to C on the second octave (weak). However, the strings can be difficult to press nearer the sound hole, so this area is not often used.
Pressing the frets changes the length of the string
The guitar has a range of nearly four octaves!

The range chart of the acoustic guitar
Musical Instrument Guide : Acoustic Guitar Contents
Structure
How the Instrument is Made
Choosing an Instrument
Trivia
- The guitar top is actually two panels stuck together
- The shape of the pick guard is dictated by the manufacturer
- Collector's item with a modern Japanese style
- Cutaways make it easier to access upper frets
- Why do the spaces between frets grow narrower down the neck?
- The order guitars and violins are strung in differs