X-ray diffraction using synchrotron radiation and polarized neutron small-angle diffraction have been used to evaluate the absolute crystallographic structure and the spin helix chirality of Mn1-x Fex Si. Contrary to previous observations we show that left- and right-handed crystals can be found for MnSi and its iron substituted analogs. The structural chirality rigorously determines the magnetic chirality of these compounds: left-(right-)handed crystalline chirality establishes left (right) handedness of the magnetic helix.