One limiting factor in swept wing design is the so-called "middle effect". If a swept wing is continuous – an oblique swept wing, the pressure iso-bars will be swept at a continuous angle from tip to tip. However, if the left and right halves are swept back equally, as is common practice, the pressure iso-bars on the left wing in theory will meet the pressure iso-bars of the right wing on the centerline at a large angle. As the iso-bars cannot meet in such a fashion, they will tend to curve on each side as they near the centerline, so that the iso-bars cross the centerline at right angles to the centerline. This causes an "unsweeping" of the iso-bars in the wing root region. To combat this unsweeping, German aerodynamicist Dietrich Küchemann proposed and had tested a local indentation of the fuselage above and below the wing root. This proved to not be very effective. During the development of the Douglas DC-8 airliner, uncambered airfoils were used in the wing root area to combat the unsweeping. Similarly, a decambered wing root glove was added to the Boeing 707 wing to create the Boeing 720.