The three-legged stool approach officially began in the 1920s when questioning an art connoisseur’s opinion, “was as naughty as inquiring about the digestive system of an opera singer…it wasn’t proper. And that was very good for the trade.”
The quote is from George L. Stout (1897–1978), a was a conservator with Harvard’s Fogg Museum of Art. (Stout is now more famously known as the inspiration for the George Clooney character in The Monuments Men.) With an inter-disciplinary group of scientists, art historians, and fellow conservators, Stout developed what is known as the three-legged stool approach to art authentication. The three-legged stool concept is now the accepted norm ethically and judicially.
Scientifically based, the three legs are:
Today, with far more scientific tools to examine paintings and access to the Internet for world-wide research, many legs can be added to the figurative three-legged stool to solidify attributions. The basic principles, however, remain constant.
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 National Academies Press, Scientific Examination of Art: Modern Techniques in Conservation and Analysis (National Academies Press, 2005), 43.
 Ibid, 42-43.