The (L)-alpha-lyxopyranosyl-(4'-->3')-oligonucleotide system-a member of a pentopyranosyl oligonucleotide family containing a shortened backbone-is capable of cooperative base-pairing and of cross-pairing with DNA and RNA. In contrast, corresponding (D)-beta-ribopyransoyl-(4'-->3')-oligonucleotides do not show base-pairing under similar conditions. We conclude that oligonucleotide systems can violate the 'six-bonds-per-backbone-unit' rule by having five bonds instead, if their vicinally bound phosphodiester bridges can assume an antiperiplanar conformation. An additional structural feature that seems relevant to the cross-pairing capability of the (L)-alpha-lyxopyranosyl-(4'-->3')-oligonucleotide system is its (small) backbone/basepair axes inclination. An inclination which is similar to that in B-DNA seems to be a prerequisite for an oligonucleotide system's capability to cross-pair with DNA.