Every theater in the world has one,
and here’s the one at LA’s Music Center:
That’s the view straight up toward the ceiling, the red light is at the edge of the balcony, right in the middle of the house so dancers can spot their turns.
I think I remember reading somewhere that in ye olden times that red light was a candle?
It’s neat to think about dancers checking in with that light throughout time and all over the world.