The principal of the wooden slit-drum is as simple as it is ingenious. A tree or a solid block of wood is hollowed out to leave a longitudinal opening on the upper side. The edges of this slit are of unequal thickness and produce two sounds of different pitch when struck. This type of instrument is known virtually all over the world: in New Hebrides, a slit drum of impressive size, standing upright like a real "tree drum," is found in many regions. The wooden linga of the Banda-Linda people of Central Africa are big-bellied drums, set on four feet, of a shape not unlike that of the buffalo. They are generally used in groups of three (or four) instruments of different size (the largest may measure up to two meters long) and form a kind of family. Each player hammers the edges of the slit with a pair of mallets ending in a ball of latex, to produce two different notes.