Mixing in a spatially periodic continuous mixer, the partitioned pipe mixer, is studied using bifurcation analysis with the help of geometric construction of periodic orbits and a knowledge of the symmetry of the mixing system. Mixing windows, where motion in the mixer is nearly globally chaotic, are found in the parameter space. Uniform ultimate mixtures are expected in the mixing windows as verified with the Poincare´ sections. The present analysis provides a better understanding of the existing experimental results.