Abstract. Let G be the automorphism group of a regular right-angled building X. The "standard uniform lattice" 0 Ä G is a canonical graph product of finite groups, which acts discretely on X with quotient a chamber. We prove that the commensurator of 0 is dense in G. This result was also obtained by Haglund (2008). For our proof, we develop carefully a technique of "unfoldings" of complexes of groups. We use unfoldings to construct a sequence of uniform lattices n Ä G, each commensurable to 0 , and then apply the theory of group actions on complexes of groups to the sequence n . As further applications of unfoldings, we determine exactly when the group G is nondiscrete, and prove that G acts strongly transitively on X.

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