The development of a special 1s pres ind marker /-j/ for the four forms so ‘to be’, do ‘to give’, vo ‘to go’, and estó ‘to be, stand’ between the 13th and the 16th centuries requires reevaluation, continuing the study by Gago-Jover (1997) and taking advantage of the new availability of extensive data bases (ADMYTE/HSMS, Corpus del español, CORDE). Understanding analogy as the essential theoretical construct for morphological change and considering the entire Romance domain, the etymological source for this marker /-j/ is argued to be a local analogy of so to its 1s preterit /'fuj/. This solution can account for the attested chronology of the change (so »soy significantly predates the other changes). Two largely independent regional developments in Castilian and Leonese provide contrasting multi-stage diffusion patterns for affecting the closed subset of the four 1s pres ind verb forms. The received opinion that /-j/ derives from the Medieval Spanish locative adverb (h)y ‘there’, must be discarded (Lloyd 1987, Penny 2002); other proposed solutions may have codetermined the change, but remain marginal. The study contributes to the debate on the architecture of morphology in linguistic theory, to the need for philological critique in dealing with historical texts, and to the elucidation of a long-standing problem of Spanish historical morphology.