open form. A structural procedure whereby the sequence and/or construction of parts of a notated work are variable. First employed by Ives, Cowell, and Grainger, but developed as indeterminacy by Cage and Earle Brown. In Boulez's 3rd pf. sonata, for example, the 5 movts. may be played in any order except the 3rd which must stay central.

open form In crystallography, crystal faces are often described in groups (‘forms’); if the form is open at each end (i.e. it does not totally enclose space), it is called an ‘open form’, e.g. the four prismatic faces of a tetragonal crystal. Compare CLOSED FORM.