This documentary about composer John Cage and choreographer Merce Cunningham, who were companions for many years until Cage’s recent death, gives a good sense of their achievements by including many fragments of their works (there’s some wonderful archival footage of Cunningham dancing from as early as 1944) and also through its relatively loose form. Director Elliot Caplan understands that these major artists cannot be pinned down: pieces of music and dance are included, but are cut together so as not to lead to any obvious conclusions or even imply a linear evolution. We hear Cunningham tell a key story from his youth about one of his dance teachers speaking to her pupils: “Quit walking in a line like a bunch of amateurs.” And indeed the work of both men has resolutely avoided obvious patterns, seeking a more open form that leaves much up to the perceiver. Both men’s work can only be fully appreciated in live performance, but this film is a good beginning. Music Box, Saturday and Sunday, July 24 and 25.