There are a variety of leaf springs, usually employing the word elliptical. Elliptical or full elliptical leaf springs, patented in 1804 by the British inventor Obadiah Elliott, referred to two circular arcs linked at their tips. This was joined to the frame at the top center of the upper arc, the bottom center was joined to the live suspension components, such as a solid front axle. Additional suspension components, such as trailing arms, would usually be needed for this design, but not for semi-elliptical leaf springs as used in the Hotchkiss drive. That employed the lower arc, hence its name. Quarter-elliptic springs often had the thickest part of the stack of leaves stuck into the rear end of the side pieces of a short ladder frame, with the free end attached to the differential, as in the Austin Seven of the 1920s. As an example of non-elliptic leaf springs, the Ford Model T had multiple leaf springs over its differential that were curved in the shape of a yoke. As a substitute for dampers (shock absorbers), some manufacturers laid non-metallic sheets in between the metal leaves, such as wood.