So-called dangling end contributions arise from the stacking of unpaired bases to an adjacent base pair. We have to distinguish two types of dangling ends: (1) interior dangles, where the unpaired base i + 1 stacks onto i of the adjacent basepair (i, j) and correspondingly j - 1 stacks onto j and (2) exterior dangles, where i - 1 stack onto i and j + 1 stacks on j. The corresponding energy contributions are denoted by  and , respectively. Within the additive energy model, dangling end terms are interpreted as the contribution of 3' and 5' dangling nucleotides: