(Fr.: ‘open’).
In medieval French music the first-time ending for a repeated musical section; the second-time ending is termed clos (Fr.: ‘closed’). The words are found particularly in the sources of 14th-century music (like the Italian Aperto (ii) and Chiuso (ii)) since songs at that time characteristically included sections that were repeated with the second ending on the tonic, or final, and the first ending on some other pitch, often a 2nd away. In many such cases the open and closed endings would be equal in length and would have the same part-writing except for the shift to a different pitch level.
The words ‘clausum’ and ‘apertum’ were applied to such endings by Johannes de Grocheo (c1300) and many subsequent theorists. In his Tractatus cantus mensurabilis (CoussemakerS, iii, 124–8) Egidius de Murino even provided instructions for the composition of secular forms in terms of the ouvert and clos cadences: for a ballade simplex there was to be an apertum and a clausum at the end of the first half but only a clausum at the end; a ballade duplex should have apertum and clausum after each half; a virelai simplex has an apertum at the end of the first half and a clausum at the end of the second half; a virelai duplex has an apertum and a clausum after each half; and a rondeau should have a clausum at the end, but at the middle it should have an apertum built on a 10th when it finishes on ut and on a 5th when it finishes on la. A similar passage in US-PHu lat.36, f.207, refers to the endings as overtum and clausulum.