Modern bathtubs have overflow and waste drains and may have taps mounted on them. They are usually built-in, but may be free-standing or sometimes sunken. Until recently, most bathtubs were roughly rectangular in shape, but with the advent of acrylic thermoformed baths, more shapes are becoming available. Bathtubs are commonly white in colour, although many other colours can be found. The process for enamelling cast iron bathtubs was invented by the Scottish-born American David Dunbar Buick.