Format is traditionally used to describe the shape or proportions of the support, for example the canvas, of a painting or other essentially flat work of art such as a relief. The two commonest traditional formats for paintings are the horizontal rectangle often referred to as landscape format and the upright rectangle known as portrait. Another traditional but less common format is the circular one known as the tondo. Square formats have sometimes been used, notably for example by JMW Turner and in the twentieth century Ad Reinhardt. The abstract painter Piet Mondrian occasionally used a square canvas hung by one corner as a diamond. From the 1960s on a much freer approach to format became evident in some of the work of artists such as Frank Stella and Ellsworth Kelly and the shape of the canvas became an important element in the composition of the work.