In this section we study the simplest non-abelian case, corresponding to two M5-branes wrapped on S 2 , or equivalently we study the reduction of the 5d U (2) theory to 4d on an interval with Nahm pole boundary conditions. The flat 4d theory is given by a map into the 2-monopole moduli space M 2 , with the action given in (4.30). For the curved space theory we find a description in terms of a sigma-model into S 1 × R ≥0 supplemented by self-dual two-forms obeying some constraints. We provide a detailed analysis of the geometrical **data** 9 The factor i is due to our conventions in which A θ is purely imaginary. 10 These transformations correspond to gauge group elements g = e iα(θ) with α(0) = 0 and α(π) = 2πn. The quantization of n is required for g to be trivial at the endpoints of the θ interval.