The Tower has a window on each face and a clerk at each one, and the clerks write in notebooks and nobody has ever seen one of them arrive or leave.

RabFour windows. Whatever they can see from four windows is what the number is.

TamsinThe yard is under all four.

RabThe yard is the only place in this school under all four at once. That is why everything happens in the yard and why it is lit until midnight.