We present a combined x-ray and neutron diffraction study of the stripe-ordered superconductor $La_{1.88}Sr_{0.12}CuO_4$. The average crystal structure is consistent with the orthorhombic Bmab space group as commonly reported in the literature. This structure, however, is not symmetry compatible with a second-order phase transition into the stripe order phase, and as we report here, numerous Bragg peaks forbidden in the Bmab space group are observed. We have studied and analyzed these

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... dden Bragg reflections. Fitting of the diffraction intensities yields monoclinic lattice distortions that are symmetry consistent with charge stripe order.