We do not fully understand Hume’s account of space if we do not understand his view of determinations of extension, a topic which has not received enough attention. In this paper, I argue for an interpretation that determinations of extension are unities in Hume’s view: single beings in addition to their components. This realist reading is reasonable on both textual and philosophical grounds. There is strong textual evidence for it and no textual reason to reject it. Realism makes perfect sense of the metaphysics of determinations of extension along Humean lines and Hume’s view of spatial relations.