Rhythm is the most basic element of music, as music cannot exist without it. It involves regular beats that are grouped into measures containing a specific number of beats. Rhythm is found in music, dance, poetry, and theater. Elements of rhythm include meter, beat, duration, and tempo. Timbre refers to the type or quality of sound produced by instruments, which can produce many different timbres. Instruments are categorized into strings, woodwinds, brass, and percussion. Rhythm gives music its propulsive energy and structure.