A double-strand chain, consisting of an uninterrupted sequence of rings, with adjacent rings having two or more atoms in common. Alternatively, a ladder chain is a double-strand chain with adjacent constitutional units joined to each other through four atoms, two on one side and two on the other side of each constitutional units.
Source:
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2295 (https://doi.org/10.1351/pac199668122287)