Nashville 12-String
A 12-string guitar actually has six pairs of strings, normally tuned as a standard 6-string guitar. However, the lowest four pairs, the E, A, D, and G, each have a string in the pair that's one octave higher. In other words, string 12 is an E3, an octave higher than string 11, an E2. String 10 is an A3, an octave higher than string 9 an A2. The same is true for the next two pairs, the D and the G. The last two pairs are unison B3s and unison E4s. So the highest string on the guitar is actually string 6 the octave G string.
12-string guitars started to be popular in the 1920s and 30s, and have been used extensively in both folk and rock music, since they sound harmonically very rich.
The Nashville 12-string (also known as a "high-strung" guitar) is actually a misnomer, since it only has 6-strings. Here the lowest 4 strings are replaced with the octave strings from a 12-string set, so from string 6 to string 1 it's E3, A3, D4, G4, B3, and E4. The Nashville 12-string has been used in both country and rock to add higher harmonics into the mix of guitars.