For example, there is a 15 minute hold planned at T-4 minutes, which occurs at L-19 minutes. Under normal conditions, these countdowns remain in sync. L- (pronounced “L minus”) refers to the days, hours, and minutes remaining in the scheduled countdown to launch, which occurs at L-0.
The last ten seconds are usually counted down aloud “Ten seconds to liftoff. “T minus Time” is a system to mark points at which actions necessary for the launch are planned – this time stops and starts as various hold points are entered, and so doesn’t show the actual time to launch.
