The microstructures of coherent , and domain walls in the orthorhombic phase of ferroelectric perovskites have been studied on the basis of Landau - Ginzburg theory for the first-order phase transitions. It is found that for the space profile of the polarization within an uncharged wall layer, the normal component always remains zero or constant, while the component parallel to the wall plane has two kinds of inhomogeneous configuration, one of the `Ising type' and the other of the `Bloch type'. Quasi-one-dimensional analytic solutions for polarization components, elastic strains, and clamping stresses are obtained for the Ising-type , and wall interfaces. The structural characteristics and physical properties of the Bloch-type walls are also illustrated and discussed. All of the theoretical results are functions of macroscopic quantities for perovskite crystals, and can be applied to real systems when these quantities are obtained from experiments.