The ferroelastic domains of the orthorhombic phase of YBa2Cu3O7-δ were obsd. in polarized light on ceramics and single crystals. By combining polarized light microscopy with the x-ray presession technique, the correlation of the orientation of the orthorhombic a- and b-axes with that of the a-b-plane bireflectance, reflection dichroism, transmission dichroism, reflection tints generated with compensators and on uncrossing of polars, as well as the orientation of etch pits was realized on ferroelastic single domains, bi-domains, and more complicated domain patterns. Four ferroelastic orthorhombic domain states were identified, at variance with former group theor. considerations, predicting only 2 states. Ensembles of lamellar domains beyond optical resoln. generate strong bireflectance with principal axes rotated by 45° relative to the true a,b-directions.