'Documenta IV', Kassel, Germany, 27 June-6 October 1968, cat. no. 1 (with title as 'Untitled (diptych)', 1967).
'Five Artists: A Logic of Vision', Museum of Contemporary Art, Chicago, 4 May-23 June 1974, no cat. no. (with title as 'Diptych, verticals flanking, red, large 1966-73').
'Object and Content: Meaning in Minimal Art', Australian National Gallery at the Drill Hall Gallery, Canberra, 24 November 1990-20 January 1991, no cat. no.