http://www.bothner.co.za/articles/nashville.shtml
http://www.wikihow.com/Tune-Your-Guitar-to-Nashville-Tuning
Nashville Tuning
Nashville tuning is a means of creating the effect of a twelve-string guitar by using a six-string guitar. Not just a tuning scheme, this method requires different strings to be used, which is why Nashville tuning is also referred to as "high-stringing" a guitar. While there are string sets available for Nashville Tuning, they are hard to find. Generally, the octave strings from 12-string sets are used. The tuning scheme is as follows: The first two strings of your guitar (high E and B) remain unchanged and the lower four strings (G through low E) are tuned an octave above standard tuning. Nashville tuning particularly shines as a means of adding a shimmering texture to rhythm tracks. Instead of using one-third of a 12-string set and throwing the rest away, a less expensive means is to buy the strings individually. The gauges for a medium set, from high E to low E, would be: .012, .016, .010, .014, .020, .030. A light set would be: .010, .014, .009, .012, .018, .027. With the exception of low E, all strings are unwound.