In the crystalline state, polymer chains are generally parallel to one another but neighbouring chains of equivalent conformation may differ in chirality and/or orientation. Chains of identical chirality and conformation are isomorphous. Chains of opposite chirality but equivalent conformation are enantiomorphous. For example, two ...$\ce{TG^{+}TG^{+}TG^{+}}$... helices of isotactic poly(propylene) are isomorphous. Isotactic poly(propylene) chains of the ...$\ce{TG^{+}TG^{+}TG^{+}}$... and ...$\ce{G^{+}TG^{+}TG^{+}T}$... types are mutually enantiomorphous. With regard to orientation, consider a repeating side group originating at atom \(A_{1}^{i}\), the first atom of the side group 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.