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The theory

Olav Torvund's Music Theory for Guitarists explains that "The perfect fourth is two and a half notes above the root," and provides audio examples of perfect fourths in the C major scale, which include C-F, D-G, E-A, G-C, A-D and B-E. The ratio of frequencies between the notes in a perfect fourth is approximately 4:3.

The inversion of a perfect fifth is a perfect fourth, and vice versa. For example, if the root of the chord C-G (a perfect fifth) is moved up an octave, it becomes G-C. As tdent notes, "There's also an old debate as to whether a note a 4th above the bass should be considered a dissonance (since it resolves to the 3rd above)."

The perfect intervals are a perfect fourth, a perfect fifth, a perfect octave, and a perfect unison, which is a chord using two of the same notes. Of these, the perfect fourth and fifth have a shared peculiarity: "If either of these are (raised) by a semitone then they are termed 'augmented'. If they are (lowered) by a semitone they are called 'diminished'." This is according to a lesson at the On-Line Guitar Archive (OLGA), which also notes that the main purpose of learning to distinguish different intervals is to develop perfect pitch.

The resources

  • Lessons on guitar chords and music theory can be found at http://www.torvund.net/guitar/Theory/ 03-intervals-continued.asp (Olav Torvund);
  • http://cnx.rice.edu/content/m10867/latest/ (The Connexions Project) offers simpler lessons with visual guides;
  • the use of different chord intervals in part-writing and melodic composition is explained at http://cctr.umkc.edu/user/bauera/Guide.html;
  • and relevant lessons from OLGA can be found at http://www.olga.net/dynamic/browse.php?local=resources/lessons/country/IntroToTheory.txt, which also features a large chord dictionary and information from Usenet guitar-related newsgroups.
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