The goal of this article is to clarify the relationship between the topos of triads and the neo-Riemannian PLR-group. To do this, we first develop some theory of generalized interval systems: 1) we prove the well known fact that every pair of dual groups is isomorphic to the left and right regular representations of some group (Cayley's Theorem), 2) given a simply transitive group action, we show how to construct the dual group, and 3) given two dual groups, we show how to easily construct sub dual groups. Examples of this construction of sub dual groups include Cohn's hexatonic systems, as well as the octatonic systems. We then enumerate all Z_{12}-subsets which are invariant under the triadic monoid and admit a simply transitive PLR-subgroup action on their maximal triadic covers. As a corollary, we realize all four hexatonic systems and all three octatonic systems as Lawvere--Tierney upgrades of consonant triads.

This final version will be published in the Proceedings of the 3rd International Conference on Mathematics and Computation in Music, in the Springer LNCS/LNAI Series. http://mcm2011.ircam.fr/ An appendix on topos theory has been added and some typos have been corrected. "Tritone Mixture" --> "Major Triad Tritone Mixture", "Skrjiabin Chord" --> "Prometheus Tritone Mixture"