Kuennapas, T. & Janson, A. (1969). Multidimensional similarity of
letters. Perceptual and Motor Skills, 28, 3-12.
Experimenters asked subjects to judge pair-wise similarities
of 28 lowercase Swedish Letters. Letters were presented in
pairs on printed sheets. Height of 'n' was 2.8 cm, height of
'p' or 'h' was 4.0 mm.
The 28 lowercase letters. This excludes the 'w' of the
English alphabet, and includes three accented vowels. Figure
1 from the article, reproduced below, shows the letters and font used.