Bluegrass music grew out of traditional string band music that formed the roots of country music. In the '40s, country music began to splinter into different directions, as honky tonk and country-pop became genres of their own. A certain segment of country musicians continued playing traditional string music. Led by Bill Monroe, these musicians adhered to the songs, structures, and conventions of string bands, but they made the music faster, harder, and more technically demanding. The result was bluegrass; the genre was named after Bill Monroe's backing band, the Blue Grass Boys. After its inception in the mid-'40s, and its popularization in the '50s, the sound had become part of country music, and there were legions of bands that followed in their footsteps. In the late '60s, a number of bluegrass groups began expanding the possibilities of the genre, much to the chagrin of many of the music's most popular artists and dedicated fans. Consequently, the new breed of bluegrass groups were dubbed progressive bluegrass, while those that adhered to the music's heritage were tagged traditional bluegrass. Over the next three decades, progressive bluegrass changed frequently, while the sound of traditional bluegrass rarely varied.