A bertsolari is a singer of bertso, a musical verse in Basque tradition. The bertolaris are often found in pairs, in which a topic is sung extemporaneously in verses alternatively, but they can stage solo or group verse sessions too. It is usually sung to a slow tempo with long or short verses and are generally dealing with various subjects. Professional bertsolaris can be found at festivities, singing improvised rhymed verses. Bertso improvisations are popular with young people at bars or special occasions.