The standard 1020 greenhouse flat with drainage slots. Can be used under other flats or directly for seed starting. All are standard size. 11" by 21". Trays and flats are made of 80% recycled scrap polystyrene and are reusable and recyclable. Compatibility: Use with Pro-Trays, Jumbo Plug Flats, Soil Block Propagation Trays, and Mini Square Plastic Pots.