The use of subordination enables language users to achieve syntactic efficiency by allowing them to connect ideas in temporal/logical relation. Although the importance of subordination has been recognized in previous research on second language (L2) writing, it has been typically assessed with global indices that measure overall ratio of subordination. In order to capture more nuanced patterns in the development of L2 writing, this study measures the sophistication of subordination, considering subordination type (adverbialization, complementization, relativization) and finiteness (finite, nonfinite). Our natural language processing analysis of 6,566 timed impromptu essays using the Subordination Sophistication Analyzer 1.0 showed that higher-proficiency L2 learners used more subordination, and, importantly, their patterns of use differed by subordination type and finiteness. Whereas the amount of adverbialization and relativization increased along with proficiency regardless of finiteness, the use of complementization increased only for nonfinite clauses. The broader impacts of this study for education and assessment are discussed.