Grant’s 1995 appearance on NBC’s “Tonight” show after a prostitution arrest, where Leno famously asked “what were you thinking?,” was seen in retrospect as a turning point in the late-night race. It drew a huge audience and propelled Leno to the top of the ratings, a spot he would not relinquish…

The final numbers won’t be out until later in the week, but there’s a strong chance that Letterman could average more viewers than the “Tonight” show in the second week of O’Brien’s new 11:35 p.m. job. That hasn’t happened since 2005, and the timing is significant: some of Leno’s old fans may be more amenable to searching for a new late-night habit during the transition period.