A central issue in the study of reading and spelling has been to understand how the consistency or frequency of letter-sound relationships affects written language processing. We present, for the first time, evidence that the sound-spelling frequency of subgraphemic elements of words (letters within digraphs) contributes to the accuracy with which these letters are produced in spelling. We report findings from 2 studies that demonstrate that letters within digraphs display differential susceptibility to error under conditions of disruption to orthographic working memory (O-WM). In the 1st, O-WM was disrupted as a result of neurological damage; in the 2nd, O-WM disruption was produced in neurologically intact, skilled spellers under dual task conditions with a shadowing task carried out during spelling. In both studies, segments with low versus high levels of sound-letter convergence, a measure of the frequency of sublexical mappings, were more vulnerable to disruption even when factors such as letter position, consonant-vowel context, and phoneme-to-grapheme mapping probability of the digraphs were controlled. These results contribute to our understanding of the internal texture of orthographic representations, providing evidence that individual letters differ in their activation strength and, as a result, in their susceptibility to error.