In this study, we demonstrate that the chiral SFG response of water around biomolecules is due to the incomplete cancellation of the responses of the symmetric and asymmetric stretching modes of water molecules. We show that the symmetric stretch response is the exact opposite of the asymmetric stretch response (Figure ). However, the symmetric and asymmetric stretch responses do not cancel because their frequencies are different due to intramolecular vibrational coupling and the variation in local environment of the two O-H groups within a single water molecule. Thus, their summation can lead to a nonzero "up-down" (or "down-up") chiral SFG signal (Figure ). This theoretical picture implies that the chiral SFG response of water provides an additional constraint to potentially improve curve fitting methods. Here, rather than fitting a separate Lorentzian curve to each O-H stretch feature, we fit water spectral responses with pairs of Lorentzian curves. Each pair of curves is with opposite phase, equal magnitude, and shifted frequencies. When applied to analyze our previously reported experimental chiral SFG spectra of b-sheet proteins, this fitting method allows us to reduce the number of free parameters and extract more reliable frequency distributions from spectra, hence directly revealing the distribution of local environments experienced by the O-H groups of water in the biomolecular hydration shell.