By 10:30 p.m. last Thursday, the Jefferson Theater was ringing loudly with the sounds of the quintessential sweet spot of a bluegrass concert. All the instruments, from mandolin to fiddle, have been played at least twice. The band has warmed up and the audience is warm from dancing or twirling. A haze has descended over the stage and hangs over the lights. The noise of the audience has been reduced to a soft hum and everything from here on out will be round after round of melodic instrumental combinations. Railroad Earth, a progressive bluegrass band out of New Jersey, reached this point early, and cast it away late, playing for a solid three hours. The Jefferson Theater is traditionally known for hosting a wide variety of marketable concerts. When it is not hosting notable high-energy electronic acts or trendy indie bands, it reels in the best names in bluegrass and Americana. Often, these bands don’t adhere to traditional genre distinctions, but rather fuse several sounds: combinations of folk, rock, bluegrass and even classical styles. Lucky for Charlottesville residents, the magic of this “newgrass,” another generic term for “progressive bluegrass,” transcends the music festival scene. In many ways, even the viral Mumford and Sons treads the line of newgrass, combining the traditional upright string bass and banjo with the electric bass guitar and keyboard. The joyful appeal of Edward Sharpe and the Magnetic Zeros, whose single “Home” soared to the top of the charts, stems from Americana and folk as well. The multi-genre concept has been at work for some time, and the term newgrass can be a misleading one. Even the most classical bluegrass artists are known to have experimented from the beginning. The banjo player Earl Scruggs, who died earlier this year, developed a unique finger-picking style himself, which soon became definitive to classical bluegrass. This is what makes bluegrass music so compelling: willingness to constantly reshape and redefine the genre. The history of bluegrass lends a great deal of credit to its artists. Mastering a bluegrass instrument, such as the mandolin, is not at all different from a chamber musician mastering the violin. When Todd Scheaffer (lead vocals, acoustic guitar) was crooning “Been Down this Road,” a common lament to a failed relationship, I was reminded how old these instruments are. Even the new electric ones have roots in the old acoustic forms. Railroad Earth does favor classical bluegrass more deliberately, but perhaps what makes Mumford and Sons popular is similar to what brought Railroad Earth to Charlottesville: a desire for old sounds with a new spin.