There are 12 keys of music. Using numbers to represent the 8 notes that makes up their respective major scales, it is easier to understand the tunings. These three are the most common open tunings mainly because they can be achieved using a standard guitar string configuration (EADGBE) They are called "open" because they produce a major chord when strummed. A major chord requires 3 notes (1,3,5) from the major scale.

Vestapol Tuning: 151351 aka Open D (DADf#ad)
Spanish Tuning: 515135 aka Open G (DGDgbd)
Open C Tuning: 151513 (CGCgce)

Any grouping of these three notes creates a "triad" (basic 3 note chord). The musical distance between two notes is called an "interval" and when these triads are strummed, the different configurations produce their own unique "voice". Two notes played at the same time are called "double stops". They can be thought of as a little chord fragments. 

Notice the 3 major chord variations found in the above tunings:
135, 351, 513
A major chord contains three intervals:
Example: 513 = 5-1, 1-3, 5-3

I routinely use these tunings on Cigar Box Guitar:
3 string: 151/GDg and 513/ADf# (Key of G and D)
4 string: 5135/Dgbd (Key of G)

With a little study of the above 6 string tunings, it is easy to realize my CBG tunings are based on these tunings. Of course, by keeping the same tone configurations, I can restring or retune to any key.

CD2 (Open G - 6 string - DGDgbd), CD 4 (G5 tuning - 3 string - GDg), and CD5 (Open G - 4 string - Dgbd) comprise a comprehensive course of applying Open G on three different instruments. Exploring in this way really opened up the fingerboard for me. After exploring CBG, I never looked at the full 6 string fingerboard the same way again.

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