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 ...TG+
... helices of isotactic poly(propylene) are isomorphous. Isotactic poly(propylene) chains of the ...TG+
... and ...G−
T... types are mutually enantiomorphous. With regard to orientation, consider a repeating side group originating at atom A1i, the first atom of the side group being Bαi. For certain chain symmetries (helical, for instance) the bond vectors b(→) (A1i/Bαi) have the same components (positive or negative) b(→)·c(→)/|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.
Purple Book, 1st ed., p. 43 (http://old.iupac.org/publications/books/author/metanomski.html)