Klezmer is one of those "you-know-it-when-you-hear-it" music genres that has a nostalgic, Old World sound. The music that we identify as klezmer today harkens back to the Jewish music played in Eastern Europe in the 19th century. The rhythms, melodies and even the selection of instruments give klezmer music a distinctive sound. It is a very eclectic style of music that has changed and evolved over the centuries, as Jews have dispersed and spread all over the world, and it has always been heavily influenced by other types of folk music that were popular in Europe, the Balkans and more recently, the United States.
Because klezmer borrows so much from other styles of music, one way to identify klezmer is by the types of instruments that are used -- as well as those that aren't. Many klezmer purists argue that one of its distinguishing characteristics is that the music doesn't feature any drums. However, today many contemporary klezmer ensembles do employ drums [Academy BJE].
In Europe, the cimbalom -- a hammered-dulcimer instrument in a trapezoidal box -- was widely used in early klezmer, dating back to the Middle Ages. Later, the violin served as the lead instrument in traditional klezmer, and it continues to be used in many klezmer revival groups. Today, the clarinet is considered to be essential to klezmer music, but it wasn't until the mid- and late-19th century that the clarinet started to take over as the lead instrument. Later in 19th century, the accordion also joined the act, as did brass instruments like the tuba, trombone and saxophone [source: Rogovoy].
Beginning in the late 1800s, large numbers of Eastern European Jews immigrated to the United States to flee political upheaval. There, klezmorim adopted the sounds of American popular music, incorporating new instruments and sounds into the klezmer repertoire.