An empty pocket: the block that is not back yet.Trays of 20, plates under them: one of each per pair.One box for a class of 30: 16 steel plates under two stacks of 8 trays, one tray and one plate per pair plus the teacher's. Each tray holds the 20-block starter set in the same order, sorted by colour, so a missing block is an empty pocket you see from the door.