Retrofocus Design
A wide-angle design with a negative front group that places the rear element further from the sensor than the focal length would allow.
In depth
An SLR needs clearance for a swinging mirror, so a 20 mm lens cannot physically sit 20 mm from the film. Retrofocus solves this by inverting the telephoto principle, at the cost of size, complexity and usually more distortion. Mirrorless cameras, with no mirror box, can use simpler symmetrical designs and often achieve better corners as a result.
Examples in practice
An SLR 21 mm retrofocus lens is large and heavy, while a rangefinder 21 mm of the same era is barely bigger than a lens cap.