Languages are known to exhibit universal restrictions on sound structure. The source of such restrictions, however, is contentious: Do they reflect abstract phonological knowledge, or properties of linguistic experience and auditory perception? We address this question by investigating the restrictions on onset structure. Across languages, onsets of small sonority distances are dispreferred (e.g., "lb" is dispreferred to "bn"). Previous research with aural materials demonstrates such preferences modulate the perception of unattested onsets by English speakers: Universally ill-formed onsets are systematically misperceived (e.g., "lba"[right arrow]"leba") relative to well-formed onsets (e.g., "bn"). Here, we show that the difficulty to process universally ill-formed onsets extends to printed materials. Auxiliary tests indicate that such difficulties reflect phonological, rather than orthographic knowledge, and regression analyses demonstrate such knowledge goes beyond the statistical properties of the lexicon. These findings suggest that speakers have abstract, possibly universal, phonological knowledge that is general with respect to input modality. (Contains 2 footnotes, 4 tables, and 2 figures.)