The standard guitar has six strings, but four-, seven-, eight-, nine-, ten-, eleven-, twelve-, thirteen- and eighteen-string guitars are also available. Classical and flamenco guitars historically used gut strings, but these have been superseded by polymer materials, such as nylon and fluorocarbon. Modern guitar strings are constructed from metal, polymers, or animal or plant product materials. Instruments utilizing "steel" strings may have strings made from alloys incorporating steel, nickel or phosphor bronze. Bass strings for both instruments are wound rather than monofilament.