Composers used to issue pieces in groups of six: Bach wrote six English Suites, six French Suites, six keyboard partitas, six Sonatas and Partitas for violin, six Suites for cello and Haydn wrote a whole lot of sets of string quartets issued in groups of six: Op 20, Op 33, Op 50, Op 76. Even Beethoven's first set of string quartets, Op 18, had six pieces. Sometimes, they would write a group of twelve, like Chopin's etudes in two sets of twelve. Really large projects might involve sets of 24, like the Chopin preludes or Bach preludes and fugues. Those sets coincide with the number of keys: there are twelve major and twelve minor keys. But why those sets of six? You know, I'm really not sure!
But one of the first to break the pattern was the Brazilian composer Heitor Villa-Lobos (1887 - 1959) who wrote not six, but five preludes for guitar. There has always been a rumor that he actually wrote six, but one was lost. But since his five preludes, a number of other composers for guitar have written groups of five, such as the Five Bagatelles of William Walton and the Five Preludes of Maximo Diego Pujol.
Today I want to post for you my recording of the Prelude No. 4 of Villa-Lobos--the one I learned first and that remains my favorite to this day. It is an unusual piece, with a haunting modal melody interspersed with quiet chords. Then there is a scintillating arpeggio section followed by the melody in high bell-like harmonics followed by a repeat of the opening. For some reason, the piece has always felt to me that it is floating in a very high place, so I have chosen photos of mountains in South America to accompany the music, with a couple of waterfalls for the quick arpeggios.