Understanding the layout of standard guitar tuning is the first step towards fretboard mastery. Knowing the underlying organization of the guitar’s six strings is a powerful tool that will instantly make the recognition and memorization of chord shapes and scalar configurations a lot easier.
To fully comprehend how the guitar strings are organized, we have to define the fundamental concept of a musical interval. An interval is the difference in pitch that exists between two notes. Intervals follow a numbering system that is based on this simple rule: the number of an interval is the number of letter names that it encompasses. For instance, the distance between C to G is a fifth because five letters exist between those two notes (C-D-E-F-G). Similarly, C to D is a second (C-D) and C to B is a seventh (C-D-E-F-G-A-B).
At first, the best way to visualize and understand any interval on the guitar is to move horizontally on a single string. Such process reveals the actual space or number of frets that separates two notes. For example, the smallest interval possible is the semitone or minor second (C-C#, C#-D, D-D#, and so on) and equals to one fret on the fretboard. Similarly, a tone or major second, which is the equivalent of two semitones, spans over two frets.
In order to understand how the guitar tuning is organized, we need to measure the difference in pitch that exists between two adjacent strings. Notice that five pairs of strings are possible, as the list below shows.
1) E-A (6th to 5th)
2) A-D (5th to 4th)
3) D-G (4th to 3rd)
4) G-B (3rd to 2nd)
5) B-E (2nd to 1st)
By using the numbering system explained earlier, we discover that all pairs of adjacent strings, with the exception of the two notes at number 4, are intervals of a perfect fourth (5 semitones or 5 frets). The only irregularity occurs between the 2nd and 3rd strings because only three letters exist between G and B, thus making it an interval of a major third (4 semitones or 4 frets). The difference of a semitone is crucially important in recognizing how chord shapes and scales must be modified in order to compensate for the irregularity intrinsic to the guitar’s tuning.
The fretboard diagrams below illustrate two intervals, an octave and a major third, and how their geometric shapes slightly change due to the guitar’s tuning irregularity. By following a similar approach, it is advisable to identify all musical intervals on the guitar's fretboard and become familiar with their configurations. This process will build a network of ‘musical coordinates’ you can start relying upon and build a lot of musical vocabulary from.