A systematic ab initio EOM-CCSD study of N-15-N-15 and N-15-H-1 spin-spin coupling constants has been carried out for a series of complexes formed from 11 nitrogen bases with experimentally measured proton affinities. When these complexes are arranged in order of increasing proton affinity of the proton-acceptor base and, for each proton acceptor, increasing order of proton affinity of the protonated N-H donor, trends in distances and signs of coupling constants are evident that are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants ((2h)J(N-N)) are positive and decrease as the N-N distance increases. All one-bond N-H coupling constants ((1)J(N-H)) are negative (K-1(N-H) are positive). (1)J(N-H) is related to the N-H distance and the hybridization of the donor N atom. One-bond (HN)-N-... coupling constants ((1h)J(H-N)) are positive (K-1h(H-N) are negative) for traditional hydrogen bonds, but (1h)J(H-N) becomes negative when the hydrogen bond acquires sufficient proton-shared character. The N-N and (HN)-N-... distances at which (1h)J(H-N) changes sign are approximately 2.71 and 1.62 angstrom, respectively. Predictions are made of the values of (2h)J(N-N) and (1)J(N-H), and the signs of (1h)J(H-N), for those complexes that are too large for EOM-CCSD calculations.