Monroe's style involved playing lead melodies in the style of a fiddler, and also a percussive chording sound referred to as "the chop" for the sound made by the quickly struck and muted strings. He also perfected a sparse, percussive blues style, especially up the neck in keys that had not been used much in country music, notably B and E. He emphasized a powerful, syncopated right hand at the expense of left-hand virtuosity. Monroe's most influential follower of the second generation is Frank Wakefield and nowadays Mike Compton of the Nashville Bluegrass Band and David Long, who often tour as a duet. Tiny Moore of the Texas Playboys developed an electric five-string mandolin and helped popularize the instrument in Western Swing music.
What keys was Monroe popular for? 
B and E