The guitarrón mexicano (the Spanish name of a "big Mexican guitar", the suffix -ón being a Spanish augmentative) or Mexican guitarrón is a very large, deep-bodied Mexican six-string acoustic bass played traditionally in Mariachi groups. Although similar to the guitar, it is not a derivative of that instrument, but was independently developed from the sixteenth-century Spanish bajo de uña ("fingernail[-plucked] bass"). Because its great size gives it volume, it does not require electric amplification for performances in small venues. The guitarrón is fretless with heavy gauge strings, most commonly nylon for the high three and wound metal for the low three. The guitarrón is usually played by doubling notes at the octave, a practice facilitated by the standard guitarrón tuning .
The guitarrón is used in Mexican Mariachi groups, which usually consist of at least two violins, two trumpets, one Spanish guitar, and a vihuela (a high-pitched, five-string guitar-type instrument), and the guitarrón. A strap is usually used to keep the instrument up and playable. The guitarrón is the principal rhythm instrument in the mariachi group, and keeps the other instruments together on beat. Guitarrón players need good left-hand strength to stop the heavy strings of the instrument and a strong right hand, specifically the index, middle finger, and thumb, to pluck two of the strings (usually a metal and a nylon string).
The back of the guitarrón is made of two pieces of wood that are set at an angle making the back shaped like a shallow letter V. This design feature increases the depth and overall size of the instrument.