  We present a base-pairing model of oligonuleotide duplex formation and show
in detail its equivalence to the Nearest-Neighbour dimer methods from fits to
free energy of duplex formation data for short DNA-DNA and DNA-RNA hybrids
containing only Watson Crick pairs. In this approach the connection between
rank-deficient polymer and rank-determinant oligonucleotide parameter, sets for
DNA duplexes is transparent. The method is generalised to include RNA/DNA
hybrids where the rank-deficient model with 11 dimer parameters in fact
provides marginally improved predictions relative to the standard method with
16 independent dimer parameters ($\Delta G$ mean errors of 4.5 and 5.4 %
respectively).
