This paper proposes a reading of Arendt's On Revolution that focuses on the spatial nature of the revolutionary phenomenon. It advances an understanding of political foundation as the ‘opening’ of a topos for political existence. I draw on the key notion of the ‘in-between’ that Arendt uses to depict the basic structure of the world as a space of coexistence and plurality. In the first part, I elucidate the anti-essentialist meaning of this term. Insofar as the ‘in-between’ refers to the constitutive distance that both separates and binds individuals together, it amounts to a social-ontological condition of political action. Yet On Revolution also shows that the grammar of the ‘in-between’ is a historical–political achievement that must be created and secured by human action on terms that are not given. In the second part, I reconstruct Arendt's account of the experience of modern revolutions and suggest that the political task of instituting and keeping such interstitial space open is enacted in the experience of rupture of time where the new can appear (the temporal abyss of freedom), and in the practice of establishing lasting institutions based on laws (the normative binding of foundation). The relation between these moments configures what I call the politics of the in-between.