The Spanish influence of Rumba is found in the songs and music. A typical song included spanish lyrics with a solo vocal section accompanied just by instruments and a call-and-response section, in which the lead singer is followed by a chorus. The content of the songs expresses various feelings. Although the song has some structure, rumba is mostly characterized by improvisions. The main instruments of Rumba are drums (conga, tumba, and quinto), sticks (claves and palitos) and cájones. Cájones are wooden boxes that were used when drums were prohibited by law between 1913 and the late 1930s. Rumba music can be heard by and identified by it's "Boom-a-tic-a" sound.