The size of a mariachi group varies depending on the availability of musicians. The usual mariachi group today consists of as many as eight violins, two trumpets and at least one guitar. Traditional mariachi guitars include the vihuela, a high-pitched, round-backed guitar that provides rhythm, and a bass guitar called a guitarrón, which also provides rhythm. Sometimes a Mexican folk harp provides bass and ornaments the melody. All are Mexican variations of European instruments. There is generally no lead singer as in other kinds of groups, with all players singing choruses and taking turns singing the lead. Often the lead singer is assigned to a certain song due to voice qualities. Mariachi vocalization shows influences from a number of styles such as bolero (a romantic style), huapango (using falsetto), son jalisciense (an aggressive style) and more. Voices must be strong to be heard over amplified instruments. Vocal style emphasizes operatic qualities, and instrumental performance demonstrates a level of virtuosity that reflects advanced musical training. Historically, mariachi groups have been made up of men, but there is growing acceptance of female mariachis.