Hot tubs and a spas are similar, but their origins differ slightly. However, both are manufactured tubs that can be installed above-ground or in-ground and do not necessarily have to remain in the same location. A hot tub is generally a wooden tub that holds enough water for two or more bathers. When manufacturers began making tubs from other materials, such as plastic and fiberglass, they started to label the tubs, spas to differentiate between their product and the older, wooden versions. Modern spas featured seating, which older, wooden tubs did not have. Jets that propelled water at users was also a feature of the spas. Fiberglass versions feature filters that help keep the tub water clean for long periods of time. Hot tubs and spas now feature many of the same amenities. They are both designed to provide relaxing hydrotherapy in exactly the same way, and both include a heater, pumps, therapeutic jets and water filters.