Guitar Chords are a group of at least 3 notes played together, this means three different notes, i.e. notes with 3 different pitches. If, for example, you select an E major chord on the guitar chord generator on this page, you can see the 3 notes E, B and G# (Ab) make up this chord. Some notes can be expressed as either sharp or flat (enharmonic spelling), the notes sound just the same but the naming of them is decided by which key the song is in. You can find out more about this in our music theory section.
A six-string guitar has five musical-intervals between its consecutive strings. In standard tuning, the intervals are four perfect-fourths and one major-third, the comparatively irregular interval for the (G,B) pair. Consequently, standard tuning requires four chord-shapes for the major chords. There are separate chord-forms for chords having their root note on the third, fourth, fifth, and sixth strings. Of course, a beginner learns guitar by learning notes and chords, and irregularities make learning the guitar difficult—even more difficult than learning the formation of plural nouns in German, according to Gary Marcus. Nonetheless, most beginners use standard tuning.
Unlike many string instruments which use a bow to create a sound, guitars are played by plucking or strumming the strings on the instrument, which vibrate to create a sound. Chords are played when the guitarist holds their fingers down on more than one individual string to change its length, which in turn changes the frequency of the vibration created - in other words, it creates a different note. Performers often use guitar tabs, rather than sheet music, when learning music: this acts as a kind of 'map' to teach guitarists where to put their fingers on the strings.