In the crystalline state, polymer chains are generally parallel to one another but neighbouring chains of equivalent @C01258@ may differ in @C01058@ and/or orientation. Chains of identical @C01058@ and @C01258@ are isomorphous. Chains of opposite @C01058@ but equivalent @C01258@ are enantiomorphous. For example, two ...TG⁺TG⁺TG⁺... helices of isotactic poly(propylene) are isomorphous. Isotactic poly(propylene) chains of the ...TG⁺TG⁺TG⁺... and ...G⁻TG⁻TG⁻T... types are mutually enantiomorphous. With regard to orientation, consider a repeating @P04472@ originating at atom \(A_{1}^{i}\), the first atom of the @P04472@ being \(B_{\alpha }^{i}\). For certain chain symmetries (helical, for instance) the bond vectors \(\overset{\rightarrow }{\mathbf{b}}(A_{1}^{i},B_{\alpha }^{i})\) have the same components (positive or negative) \(\frac{\overset{\rightarrow }{\mathbf{b}}\cdot \overset{\rightarrow }{\mathbf{c}}}{|\overset{\rightarrow }{\mathbf{c}}|}\) along the c axis for every \(i\). Two equivalent (isomorphous or enantiomorphous) chains in the crystal lattice, having identical components of the bond vectors along c, both positive or both negative, are designated isoclined; two equivalent chains having bond vectors along c of the same magnitude but opposite sign are designated anticlined.