It is my understanding that the tenor guitars were developed as an instrument that the tenor banjo player could easily play when a guitar sound was desirable. Tenor banjos were common in vaudeville type acts and other types of bands (see above) in the early parts of the 20th century. So its origins were as a 4 string guitar, tuned as a banjo. When everybody started playing folk guitar in the 50's, it became common for the tenor guitars around to be strung up and tuned to match the 6 string guitar so all those guitar players could easily use it. So the tuning rules are pretty much use whatever you are most comfortable with. As mentioned above, Nick Reynolds has been probably the most visible practitioner of the tenor guitar in the last 40 years. Nick grew up in a 4 string tradition playing Martin ukuleles. He originally played a Martin 0-17T with mahogany top back and sides. It was soon replaced by a Martin 2-18T that was used on many of the Trio's early albums. That guitar had been built in 1929. In 1959 he added a martin 0-18T to his performing and recording collection. In 1961 he had the 1929 2-18T converted to an 8 string version. Later he had another 0-18T also converted to 8-string. He usually used thick felt ukulele picks on the 4-string models and flexible plastic picks for the 8-string. The 8-string didn't show up much in photo shoots. The one exception being a group of photos of the "Guard" trio taken at a beach setting. One set had a wooden pier in the background and a blond girl on the pier or sitting in the sand. The other set had white and yellow sailed sail boards in the background. Those photos became album covers for reissues of Trio material under the titles of "Tom Dooley" and "Scarlet Ribbons"