There are two: the cyclic group (C10) and the dihedral group (D10).

You can't. Rik is messing with your mind.

Dihedral is the angle between the mounted position of the wings and the horizontal axis. A dihedral allows better in flight stability.

Dihedral usually makes an aircraft more stable in attitude.

Dihedral is the angle that the wing diverges from horizontal. Most planes have a positive dihedral, meaning the wings angle up slightly. This gives the plane stability in flight. Acrobatic and fighter planes tend to have a negative dihedral (wings droop) which allows them to manoeuver quickly.