The earliest application of this method was in 2004, when the tension between late neutral gas indicated by quasar spectra and early reionization suggested by CMB results was strong. The detection of Lyman alpha galaxies at redshift z=6.5 demonstrated that the intergalactic gas was already predominantly ionized at an earlier time than the quasar spectra suggested. Subsequent applications of the method suggested some residual neutral gas as recently as z=6.5, but still indicate that a majority of intergalactic gas was ionized prior to z=7.