The playing of polyrhythms is one of the most universal characteristics of Sub-Saharan music, in contrast to polyphony in Western music. Several uniquely designed instruments have evolved there over time to facilitate the playing of simultaneous contrasting rhythms. The mbira, kalimba, Kora, Ngoni and dousn'gouni are examples of these instruments which organize notes not in the usual single linear order from bass to treble, but in two separated rank arrays which allows additional ease in playing cross rhythms. The continuing influence of this principle can be seen in the 20th century American instruments the gravi-kora and gravikord which are new modern examples.