These trays were hung on a radiator when the radiators in the house were fully on and the air in the house was very dry. and filled with water. Due to the heat from the radiator, the water slowly evaporated, and so the humidity in the house was maintained. On the back are two felt pads that have been flattened by a previous owner. The trays has a 'middle partition' in which there is a hole at the top. For example, the tray can be hung on the radiator with, for example, a sturdy iron wire. It is a nice retro item for an interior decorated in the style of the 60s and 70s. The condition of the tray is in a good used vintage condition