number of "keys". In some of these schemes, the two strands of the gut that form the fret are separated so that the player can finger a slightly sharper or flatter version of a note (for example G versus A) to suit different circumstances. Treatises Descriptions and illustrations of viols are found in numerous early 16th-century musical treatises, including those authored by: Sebastian Virdung: Musica getutsch, 1511 Hans Judenkunig: Ain schone kunstliche Vunderwaisung, 1523 Martin Agricola: Musica instrumentalis deutsch, 1528 Hans Gerle: Musica Teusch (or Teutsch), 1532 Both Agricola's and Gerle's works were published in various editions. There were then several important treatises concerning or devoted to the viol. The first was by Silvestro Ganassi dal Fontego: Regola Rubertina & Lettione Seconda (1542/3). Diego Ortiz published Trattado de Glosas (Rome, 1553), an important book of music for the viol with both examples of ornamentation and pieces called Recercadas. In England, Christopher Simpson wrote the most important treatise, with the second edition being published in 1667 in parallel text (English and Latin). This has divisions at the back that are very worthwhile repertoire. A little later, in England, Thomas Mace wrote Musick's Monument, which deals more with the lute but has an important section on the viol. After this, the French treatises by Machy (1685), Rousseau (1687), Danoville (1687), and Etienne Loulie (1700) show further developments in playing technique. Popularity Viols were second in popularity only to the lute (although this is disputed), and like lutes, were very often played by amateurs. Affluent homes might have a so-called chest of viols, which would contain one or more instruments of each size. Gamba ensembles, called consorts, were common in the 16th and 17th centuries, when they performed vocal music (consort songs or verse anthems) as well as that written specifically for instruments. Only the treble, tenor, and bass sizes were regular members of the viol consort, which consisted of three, four, five, or six instruments. Music for consorts was very popular in England in Elizabethan times, with composers such as William Byrd and John Dowland, and, during the reign of King Charles I, John Jenkins, William Lawes and Tobias Hume. The last music for viol consorts before their modern revival was probably written in the early 1680s by Henry Purcell. Perhaps even more common than the pure consort of viols was the mixed or broken consort (also called Morley consort). Broken consorts