To expand our understanding of script-general and script-specific principles in the learning of letter names, we examined how three characteristics of alphabet letters -- their frequency in printed materials, order in the alphabet, and visual similarity to other letters -- relate to children's letter-name knowledge in four languages with three distinct scripts (English [N = 318; M age = 4.90], Portuguese [N = 366; M age = 5.80], Korean [N = 168; M age = 5.48], and Hebrew [N = 645; M age = 5.42]). Explanatory item response modeling analysis showed that the frequency of letters in printed materials was consistently related to letter difficulty across the four languages. There were also moderation effects for letter difficulty in English and Korean, and for discriminatory power of letters in Korean. The results suggest that exposure to letters as measured by letter frequency is a language-general mechanism in the learning of alphabet letters. [This is the online first version of an article published in "Scientific Studies of Reading."]