where ℓ1 is the length of the unpaired strand between i and k and ℓ2 is the length of the unpaired strand between l and j. Symmetry of the energy model dictates (i, j; k, l) = (l, k; j, i). If ℓ1 = ℓ2 = 0 we have a (stabilizing) stacked pair, if only one of ℓ1 and ℓ2 vanish we have a bulge. For multiloops, finally we have an additive energy model of the form  = a + b × β + c × ℓ where ℓ is the length of multiloop (again expressed as the number of unpaired nucleotides) and β is the number of branches, not counting the branch in which the closing pair of the loop resides.