The music, however, is commonly very rhythmic with a strong importance placed on the use of drums. These drums exhibit characterising features such as polyrhythm (two contrasting rhythms being combined simultaneously) and ostinato (a single pattern being repeated constantly). African vocal music often exhibits built up layers of harmony, usually in thirds, as well as a distinctive ‘call and response’ figures where there is a direct dialogue between two singers or groups of singers. Some commonly used traditional African instruments include the talking drum (an hourglass-shaped drum that can change pitch), the djembe (a goblet-shaped drum), the kora (a plucked string instrument that typically has 21 strings), the mbira (a small thumb piano), and the balafon (a xylophone-type instrument).