Indicators of letter visual similarity have been used for controlling the design of empirical and neuropsychological studies and for rigorously determining the factors that underlie reading ability and literacy acquisition. Additionally, these letter similarity/confusability matrices have been useful for studies examining more general aspects of human cognition, such as perception. Despite many letter visual-similarity matrices being available, they all have two serious limitations if they are to be used by researchers in the reading domain: (1) They have been constructed using atypical reading data obtained from speeded reading-aloud tasks and/or under degraded presentation conditions; (2) they only include letters from the English alphabet. Although some letter visual-similarity matrices have been constructed using data gathered from normal reading conditions, these either are based on old fonts, which may not resemble the letters found in modern print, or were never published. For the first time, this article presents a comprehensive letter visual-similarity/confusability matrix that has been constructed based on untimed responses to clearly presented upper- and lowercase letters that are present in many languages that use Latin-based alphabets, including Catalan, Dutch, English, French, Galician, German, Italian, Portuguese, and Spanish. Such a matrix will be useful for researchers interested in the processes underpinning reading and literacy acquisition.
rate the letter pairs on the basis of visual similarity, rating them from 1 (not at all similar) to 5 (perfect match)