Establishing a quantitative relationship between backbone-backbone hydrogen-bond (H-bond) length observed in protein crystal structures and recently observed (3h)J(NC') couplings across such bonds is Limited:by the coordinate precision of the X-ray structure. For an immunoglobulin binding domain of streptococcal protein G, very high-resolution X-ray structures are available. It is demonstrated that over the small range of N-O H-bond lengths (2.8-3.3 Angstrom) for which (3h)J(NC') couplings are observable, the 32 measured (3h)J(NC') values can be fit to: (3h)J(NC') = -59000 exp(-4R(NO)) +/- 0.09 Hz, or R-NO = 2.75 - 0.25 In(-(3h)J(NC')) +/- 0.06 Angstrom. Backbone amide to side-chain carboxyl hydrogen bonds were also investigated, and the measured (3h)J(NC') values tend to be smaller than expected from their crystallographically determined H-bond lengths. The sign of (3h)J(NC'), determined from a zero-quantum/double-quantum experiment,is found to be the same as that of the (1)J(NH) coupling, i.e., negative.