A Mexican string instrument (literally 'sixth bass') with 12 strings in 6 double courses (a closely related instrument is the bajo quinto which has 10 strings in 5 double courses).
In the 17th and 18th centuries, Mexican artisans built several types of instruments with double strings in from 3 to 8 courses, influenced by their Spanish ancestors. The manufacture of bajo quinto and sexto reached a peak in quality and popularity in the 19th century in central and southern Mexico. Near the end of the 19th century, the bajo sexto began to migrate northwards, where it became a popular instrument for weddings and dances.
The 1930s saw the rise of conjunto music and the instruments of choice for this developing style were accordion and bajo sexto. At this time the bajo sexto functioned primarily as a bass instrument, providing a strong rhythmic foundation supporting the solo accordion. In the late 1940s, string bass (and later, electric bass) was added, and in the 1950s drums, completing the modern conjunto ensemble. The inclusion of bass and drums freed the bajo sexto from exclusively rhythmic bass duties and bajo players began experimenting with chords, counter rhythms and melodic lines.
The bajo sexto physically looks like a 12-string guitar. However, there are important differences: the body is usually a bit deeper, the neck is shorter, joining the body at the 12th fret, and the strings are thicker. Older instruments tend to have a larger body, while modern instruments are more guitar-like. Modern instruments also frequently have a cutaway adjacent to the neck, allowing easier access to higher playing positions for the left hand.
Since the instrument is tuned an octave below a standard guitar, the body on some instruments is not large enough for the lowest E to resonate well, and many players removed the 6th course, playing on only 10 strings (5 courses). Luthiers eventually picked up on this practice and began leaving off the low E course during construction, producing instruments with only 5 courses — bajo quintos.