There are lots of banjo-player jokes. One goes “You can drop a shoe on the banjo and make music,” due to the “open G” tuning that is standard among bluegrass banjoists. Unlike a guitar or mandolin, which must be fretted to make a proper chord, merely strumming the open strings of a banjo (or dropping a shoe on it) will produce a G major chord. This tuning of D-B-G-D-g (from first to fifth string) provides two G notes an octave apart, two D notes an octave apart, and one B note, thus filling in the three notes of a G major triad. While the vast majority of bluegrass banjo work is performed with this tuning, alternate tunings are sometimes employed. The “drop C” tuning of D-B-G-C-g was considered the standard tuning in the classical banjo era around 1900 and is sometimes used by bluegrass players to provide a low root note when playing in the C position. Bluegrass banjo standards traditionally performed in C tuning include Home Sweet Home and Farewell Blues as recorded by Earl Scruggs.
The next most-common tuning for bluegrass banjo is open D, either D-A-F#-D-F# or D-A-F#-D-a (the fifth string can either be tuned down one fret from G to provide the third note of the D major triad, or tuned up two frets to provide the fifth note). Reuben and John Henry are traditionally played in this tuning, and Ron Block has used it to great advantage in his work with Alison Krauss and Union Station. Rare alternate tunings include G minor (D-Bb-G-D-g) as used by Ben Eldridge in his original instrumental Appalachian Rain recorded with The Seldom Scene, and D minor (aDFAD) as featured by Earl Scruggs in Nashville Blues.
The world of old-time clawhammer banjo makes use of a much greater variety of tunings including “double C” (D-C-D-Cg) and open C (E-C-D-C-g), to name just two. . . but that’s a subject for another post!