Alastair Grieve writes in the catalogue to Hill's 1983 retrospective at the Hayward Gallery: ‘Hill's art of the last decade and a half has reflected his mathematical interests. The series of reliefs called Parity Studies are an example. One of these (no. 80) was shown as early as 1969 in his second Kasmin Gallery exhibition where it hung with a group of Partitions and Quadrant reliefs and the Screens. Several others were made in the first half of the seventies and three were exhibited at the Hayward Gallery in the winter of 1975. All use the motif of the familiar 120° angle but now laid flat rather than projecting into space. In the Parity Studies the 120° units are combined into eight groups of three. They do not form closed hexagons, but are linked as ‘trees' in horizontal and vertical compositions. Each asymmetric group of three 120° units is kept separate. The materials used for this series vary. The version shown at the 1969 Kasmin Gallery exhibition was made from white-faced engraver's laminated plastic with the design cut in matt black. The engraving was done by professionals with extreme precision. Hill makes much use of this technique in the next decade […] Here it is used to produce a very forceful image with the same carrying power as some London Underground signs […] Other versions of the Parity Studies were made from paint on plastic, white on black or coloured so as to distinguish the units, and from aluminium strip on sheet plastic, or stainless steel. They have square formats and are hung as lozenges.'