The primary melody of a Mariachi song is usually carried by the lead singer and a violin. The violin is played in a traditional fashion, comparable to the way in which it is played in classical music. Mariachi music is often fast-paced and high in pitch, making the violin most suitable for the job of bringing the melody to the top.
Backup singers cover the harmony. In addition, a stringed instrument known as a vihuela is played to back up the harmony singers. Finally, a guitar strung with nylon strings backs up the harmony singers and the vihuela. Depending on the song, the guitar may break from harmony to carry the lead melody or perform the introduction to a song. The guitarrón makes the rhythm complete by taking the bass line; this is often considered the most important instrument in the group.
While not explicitly necessary for a smaller Mariachi band, most performing bands have a couple of horns. The most common is likely the trumpet, which is usually found in numbers of two or three. Sometimes each trumpet will enhance a certain part of a song, while at other times trumpets are given individual and competing lines.
A small Mariachi band found in a restaurant or at a family gathering will usually have a guitarrón, a guitar, a violin and a vihuela. A full-sized band will have all of those instruments with the addition of four or five more violins and at least two trumpets. Some songs may require a flute, accordion or organ, but these instruments are generally only used on songs that specifically require them.