GUITAR CHORDS

A guitar chord is a chord, a collection of tones usually sounded together at once, played on a guitar, a type of chromatically fretted string instrument.
Chord voicings designed for the guitar can be optimized for many different purposes and playing styles. Guitar chords can be composed of notes played on only a few strings at a time, whether occurring on adjacent strings or not, or on all the strings.
The instrument is generally very capable and versatile for chording purposes, but it does exhibit some differences with other instruments. Most guitars only have six strings, which means that for the very largest of chord-voicings it's often necessary to drop or omit one or more tones from the chord; this is typically the root or fifth. The layout of notes on the fretboard sometimes demands that the notes in a chord do not run in tonal order, or makes possible a chord which is composed of more than one note of exactly the same pitch. Many chords can be played with exactly the same notes in more than one place on the fretboard.
Guitars can vary both in the number of strings they have, and in the way they're tuned. Most guitars used in popular music have six-strings and are tuned (from the lowest pitched string to the highest): E-A-D-G-B-E. The internal intervals present among adjacent strings in this tuning can be written 4-4-4-3-4 (being mostly perfect fourth intervals plus one major third interval near the middle). Conventionally, the string with the highest pitch (the thinnest) is called the first string, and the string having the lowest pitch is called the sixth.


Chord diagrams:

Guitar chords can be represented in standard musical notation, tablature (frequently referred to as tab), or in chord diagrams, which are shown below.
There are several symbols used in the chord diagram format:

Vertical lines represent the guitar strings. The line on the left represents the sixth (or thickest) string on the guitar.

Horizontal lines represent frets on the guitar. Unless a different fret number is indicated on the right-hand side of the diagram, the top horizontal line represents the nut (or "zeroth" fret).

An X above a vertical line indicates a string that is not played.

An O above a vertical line indicates an open string (a string that is played without being fretted).

A filled circle on a vertical line indicates the position in which a string is fretted to play a note.

A filled circle and square on a vertical line also indicates the position in which a string is played, and that the note is the root note or an octave of it.

A curved line is used to indicate a barre, which is a single finger used to hold multiple strings down at once.

Numbers beneath certain string indicate the finger number that is usually used to play this note. (One represents the index finger, two is the middle finger, etc.).

Note that the chord diagrams have to be reversed for left-handed guitars and guitarists.

Fret numbers

A shorthand for chord diagrams is to simply give the fret numbers of each chord. For example, this might be given as [X 3 2 0 1 0] in the case of the C major chord which is pictured below. The leftmost character gives the fret number of the sixth string (in this case X means it is not played) and the rightmost character gives the fret number of the first string (in this case open).

Six-string guitars with standard tuning

Guitar chords take advantage of the intervals between the strings, which in each case are perfect fourths excepting the interval between the B (second) and G (third) strings, which is a major third. To go above standard tuning it is common in many forms of hard rock as well as metal to use drop D tuning. This requires the player to change the low E string tuning to that of a D note.