José Guadalupe Guzmán playing the bajo sexto.
Bajo sexto (Spanish: "sixth bass") is a Mexican string instrument with 12 strings in 6 double courses, while bajo quinto has 10 strings in 5 double courses.
These instruments are used primarily in norteño music of northern Mexico and across the border in the music of south Texas known as "Tex-Mex", "conjunto", or "música mexicana-tejana".
The bajo sexto sound provides a strong rhythm in the lower pitched end of a Conjunto band and also provides a strong projection of chord changes across songs.
Bajo sextos are traditionally tuned in fourths, what an anglophone guitarist would call all fourths tuning: E,A,D,G,C,F (from lowest to highest string).
From bajo sexto derives the bajo quinto. The manufacture of the bajo sexto reached a peak in quality and popularity in the 19th century, in the states of Guerrero, Morelos, Puebla, Oaxaca and Tlaxcala.
Bajo quintos are tuned the same: A,D,G,C,F (as above). 
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