Ensembles of clay pots beaten with a soft pad are common; they are sometimes filled with water. Although normally tuned, untuned examples are sometimes used to produce a bass rhythm. Hollow logs are also used, split lengthways, with resonator holes at the end of the slit. They were traditionally used to communicate over great distances. Various bells are a common part of royal regalia, and were used in secret societies. They are usually made of iron, or in Islamic orchestras of the north, of bronze. Struck gourds, placed on a cloth and struck with sticks, are a part of women's music, as well as the bòòríí cult dances. Sometimes, especially in the north, gourds are placed upside-down in water, with the pitch adjusted by the amount of air underneath it. In the south-west, a number of tuned gourds are played while floating in a trough. Scrapers are common throughout the south. One of the most common types is a notched stick, played by dragging a shell across the stick at various speeds. It is used both as a women's court instrument and by children in teasing games. Among the Yoruba, an iron rod may be used as a replacement for a stick. Rattles are common, made of gourds containing seeds or stones are common, as are net-rattles, in which a string network of beads or shells encloses a gourd. Rattles are typically played in ritual or religious context, predominantly by women. Drums of many kinds are the most common type of percussion instrument in Nigeria. They are traditionally made from a single piece of wood or spherical calabashes, but have more recently been made from oil drums. The hourglass drum is the most common shape, although there are also double-headed barrel drums, single-headed drums and conical drums. Frame drums are also found in Nigeria, but may be an importation from Brazil. An unusual percussion instrument is the udu, a kind of vessel drum. This instrument is very essential in most African countries. String instruments The musical bow is found in Nigeria as a mouth-resonated cord, either plucked or struck. It is most common in the central part of the country, and is associated with agricultural songs and those expressing social concerns. Cereal stalks bound together and strings supported by two bridges are used to make a kind of raft zither, played with the thumbs, typically for solo entertainment. The arched