While the shaped canvas first challenged the formalized rectangular shape of paintings, it soon questioned the constraints of two-dimensionality. According to Donald Judd in his "Complete Writings": 'The main thing wrong with painting is that it is a rectangular plane placed flat against the wall. A rectangle is a shape itself: it is obviously the whole shape; it determines and limits the arrangement of whatever is on or in it". In 1964, the Solomun R. Guggenheim Museum organised the definitive exhibition 'The Shaped Canvas" curated by Lawrence Alloway. Lucy Lippard noted that this show focused exclusively on paintings with a "one -sided continuous surface" In 1965, Frank Stella and Frank Geldzahler confronted this definition of the shaped canvas by introducing three-dimensional shaped canvases by artists Charles Hinman and Will Insley in their seminal group show "Shape and Structure" at Tibor de Naguy in New York. The invasion of the third dimension by paintings was an important development of the shaped canvas as it questioned the frontier between painting and sculpture.