  We present a new formalism to describe the ratios and profiles of emission
lines from hydrogen in Balmer-dominated shocks. We use this model to interpret
the measured widths and ratios of broad and narrow H-alpha, H-beta and Ly-alpha
emission lines in supernova remnants (SNRs). Our model results agree fairly
well with those obtained previously by Chevalier, Kirshner & Raymond (1980) and
are consistent with observations of several SNRs. The same model fails to
account for the ratio of broad to narrow line emission from the reverse shock
in SNR 1987A as observed by Heng et al. (2006). We suggest that this
discrepancy between theory and observation results from a faulty assumption
that Balmer-dominated shocks can be treated as sharp discontinuities. If the
spatial structure of the shock transition zone is taken into account, the
predicted ratios of broad to narrow line emission in most SNRs will change by
modest factors, but the ratio in SNR 1987A will increase substantially.
Significantly greater shock velocities will be required to account for the
observed full widths at half-maximum of the broad emission lines in most SNRs.
