Native American music is unique in the world in having almost no string instruments. The fretless, single-string violin of Mexico's Seri Indians is therefore often remarked on in studies of Native American music. Most scholars assume that the Seri violin descended from the European violin. The Spanish introduced a variety of string instruments to Mexico following the conquest (1519), and these subsequently spread widely in many forms. However, Helen Roberts questions other scholars' arguments that Mexican string instruments could not have preconquest origins simply because they are not mentioned by early Spanish sources and do not appear in indigenous Mexican illustrations. Roberts argues that because of their low volume, string instruments would not have been used in loud group contexts such as those depicted in Mexican manuscripts.