The ribose zipper is a tertiary interaction formed by consecutive hydrogen-bonding between the backbone ribose 2′-hydroxyls from two regions of the chain interacting in an anti-parallel manner.
Reference: Cate,J.H., A.R.Gooding, E.Podell, K.Zhou, B.L.Golden, C.E.Kundrot, T.R.Cech, and J.A.Doudna. 1996. Crystal Structure of a Group I Ribozyme Domain: Principles of RNA Packing. Science 273:1678-1685.