As already stated, the perfect-fifths (P5) interval is the most harmonious, after the unison and octave intervals. An explanation of human perception of harmony relates the mechanics of a vibrating string to the musical acoustics of sound waves using the harmonic analysis of Fourier series. When a string is struck with a finger or pick (plectrum), it vibrates according to its harmonic series. When an open-note C-string is struck, its harmonic series begins with the terms (C,C,G,C,E,G,B♭,C). The root note is associated with a sequence of intervals, beginning with the unison interval (C,C), the octave interval (C,C), the perfect fifth (C,G), the perfect fourth (G,C), and the major third (C,E). In particular, this sequence of intervals contains the thirds of the C-major chord {(C,E),(E,G)}.[4]
The previously discussed I-IV-V chord progressions of major triads is a subsequence of the circle progression, which ascends by perfect fourths and descends by perfect fifths: Perfect fifths and perfect fourths are inverse intervals, because one reaches the same pitch class by either ascending by a perfect fourth (five semitones) or descending by a perfect fifth (seven semitones). For example, the jazz standard Autumn Leaves contains the iv7-VII7-VIM7-iiø7-i circle-of-fifths chord-progression;[80] its sevenths occur in the tertian harmonization in sevenths of the minor scale.[81] Other subsequences of the fifths-circle chord-progression are used in music. In particular, the ii-V-I progression is the most important chord progression in jazz music.

Once you know the basic chords, it might be easier to think of them the way the function inside a key. For example, when in the key of E, the E (I) is called the Tonic. It's what all the other chords want to get to—which is what helps give western music its sense of motion. The A (IV) in the key of E functions as the Subdominant—it's sort of a passive in-between, just as happy to continue forward, as to relax back to the Tonic. The Dominant is just what it sounds like: it leads you where it wants to go. In the key of E, that role is filled by the B (V), and will definitely make your brain want to get back to the Tonic! When you get more familiar with the chords, and want to sketch out a tune, try writing it as I-IV-V (or variations of that) instead of E-A-B. It will make it much easier to transpose when you find out your singer cannot sing in the original key!

The black dots found on the chord diagrams tell you which fret to press down, and on which string and with which numbered finger. Additionally, you will sometimes see circles above the nut. These circles tell you to play that open string without pressing any fret. So using what we know so far about strings, finger numbers, and dots, let’s check out that same chord diagram again:
My personal opinion on the topic, as one musician to another, is that the best thing you can possibly do when trying to figure out which string to go with is to try out as many different brands and types of guitar strings as you can. Strings are cheap enough that most people are going to be able to afford to experiment, and the truth of the matter is that you’re probably not going to really know what works best for you until you have hands on experience.
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