Thumb's rule tested: visual angle of thumb's width is about 2 deg
O'Shea, RP 1991, 'Thumb's rule tested: visual angle of thumb's width is about 2 deg', Perception, vol. 20, no. 3, pp. 415-418.
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Measures of the size of various hand parts and their viewing distance when held at arm's length were made on one hundred and eighteen undergraduate students. A simple rule of thumb can be confirmed: Visual angle of the width of the thumb held at arm's length is about 2 deg. The thumbnail subtends about 1.5 deg and the index fingernail about 1 deg in width, when both are held at arm's length. These figures are good approximations for males and females, although a significant, direct, linear relationship exists between hand-part size and the visual angle of the part at arm's length.