as quoted by Lucy R. Lippard, in 'Hommage to the Square', Art in America, July-August 1967, p. 55
This quote is one of the most frequently quoted statements of Agnes Martin. A later variation by her: 'The rectangle is pleasant, whereas the square is not'; Agnes Martin is than 89 - quoted in A House Divided: American Art Since 1955, Anne M. G. Wagner Univ. of California Press 2012, p. 263
1960's
“The little rectangle contradicts the square. And the square is authoritative... The rectangle is pleasant, whereas the square is not.... It's too stiff, too authoritative. My paintings are made up of little rectangles, not little squares There was a scholar who dug up a Tantric drawing that was just like my grid, and it was made of rectangles, too, just exactly like mine... I was surprised. I didn't think anybody had made a grid quite like that.”
1980 - 2000, Perfection Is in the Mind', 1995
Help us to complete the source, original and additional information
Agnes Martin 48
American artist 1912–2004Related quotes
Kenneth Noland, pp. 23-24
Conversation with Karen Wilkin' (1986-1988)

as quoted in "Ellsworth Kelly", John Coplands; Harry N. Abrams, New York, 1971
1969 - 1980
Robert Barry (1980) in: Alexander Alberro (2003). Conceptual Art and the Politics of Publicity. Alberro noted: "Barry has since discussed the way in which this painting accented the structural support..."

Source: https://www.theverge.com/22588022/mark-zuckerberg-facebook-ceo-metaverse-interview

Question: How did the mobiles start?
1950s - 1960s, Excerpt, Interview with Alexander Calder (1962)

Source: 1960s, Jours effeuillés: Poèmes, essaies, souvenirs (1966), p. 288, Arp refers in this quote to the structure in the early watercolor paintings by his wife Sophie Taeuber.

“A circle looks at a square and sees a badly made circle.”
Source: Authority
interview by Suzan Campbell, May 15, 1989; transcript in 'Archives of American Art', The Smithsonian Institution, Washington, D.C.
One of her first grid paintings she made in New York in 1964, it was [ https://www.moma.org/collection/works/78361 titled 'The Tree']. Martin often described this painting as her first grid. In fact, she had been making them since at least the beginning of 1960's
1980 - 2000