This work focuses on investigating why function word frequency, a commonly used metric in stylometry research, performs so well in distinguishing academic disciplinary writing. How does function word frequency fit with and enrich our broader theoretical conceptions of disciplines as social constructs with particular “ways of knowing and doing” (Carter, 2007) and as performing kinds of “social action” (Miller, 1984)? More generally speaking, how can focusing on frequent language-level features that corpus work reveals help us to better articulate how rhetorical action is accomplished through specific linguistic resources? I focus my analysis on function words that contribute to the difference between the humanities and sciences broadly, and ask how those words support particular types of textual moves that undergird the production of knowledge in the disciplines.