A number of Brother Acts began popping up in the 1930s in response to the rough, raw, homespun country sounds of the previous decade. Their music was more refined and professional than that of their predecessors, and was marked by gorgeous harmony vocals and spare acoustic guitar accompaniment. The brother concept made sense: having spent all those years growing up together, brothers had plenty of time to work on their singing to develop Close Harmony. The similarity of vocal timbre and phrasing made the harmonies eerily close, sometimes making it hard to tell where one voice began and the other ended. The Delmore Brothers, the Monroe Brothers (which included Bill Monroe in his pre-bluegrass days), the Blue Sky Boys, and many others became very popular, especially on the radio. The Delmore Brothers were one of the earliest star groups on the Grand Ole Opry. During the '50s, the gifted Louvin Brothers-who continue to influence legions of Alt-Country bands -- brought harmonic singing to new heights, as did the Everly Brothers in the '60s. Brother Acts still crop up now and again in contemporary country music, including the '90s duo Brother Phelps.