The short, high 5th string
On a 5-string banjo, the 5th string is short: its peg sits halfway up the neck. It is the highest string; played open with the thumb, it rings like a drone, a hallmark of bluegrass.
The head and the floating bridge shift with humidity: check your tuning every session, and the bridge position if fretted notes sound out of tune.
Open G, double C and open D
In open G, the open strings form a major chord: it is the standard bluegrass tuning. Double C and open D are common in old-time music to play in other keys while keeping ringing open strings.