Does bilingualism bring about structural similarity between the languages in contact? The convergence evaluation metric illustrated in this chapter relies on appropriate data – speech corpora from a well-established bilingual community and from monolingual benchmarks – and a replicable method – diagnostic differences between languages pivoted on the probabilistic structure of internal variation. For variable subject expression, one diagnostic difference lies in prosodic position: the variable context for null subjects in English, outside coordinate clauses, is restricted to verb-initial prosodic units, which, conversely favor pronominal subjects in Spanish. A second quantitative measure is found in accessibility: the effect of coreferentiality and clause linking with the preceding subject is stronger in English than in Spanish. On both measures, bilinguals’ English and Spanish line up with their respective monolingual counterparts and, most remarkably, are different from each other, refuting morphosyntactic convergence. When both languages are in regular use, bilingualism is compatible with continuity rather than change, being best characterized as alternation between, not mixing of, languages.