Bluegrass is a style of southern string band music developed by Bill Monroe in the 1930s and 1940s. Bill Monroe's records with the Blue Grass Boys were so popular on the radio that announcers began using the term "bluegrass music." Typical instrumentation is fiddle, banjo, mandolin, guitar, dobro, and bass. Bluegrass comes from mountain, gospel, and blues music, and generally involves a traditional, often melancholy, subject. It is played by an acoustic string band, which may be augmented by traditional instruments, such as the harmonica, mouth harp, accordion, jug, and spoons. Bluegrass is frequently characterized by virtuoso string playing, as with Monroe playing the mandolin, or Earl Scruggs the banjo; and the mandolin, following Monroe's example, is a prominent part of most bands. The tempo of the music is frequently fast, highlighting the skill of the performers. A striking feature of bluegrass is the intensely high and pure tone of lead vocalists, particularly in songs of longing or loss, as heard in the singing of Ralph Stanley, Alison Krauss, and, of course, Monroe himself.