The axial plane of a fold is the plane which divides the fold into two symmetrical halves. The axis of a fold is defined as the intersection of the axial plane with the top of a particular stratum. The bearing of the fold axis is the azimuth of the axis projected vertically onto the horizontal plane. Bearing is always measured in the down plunge direction. Therefore, the bearing may fall in any quadrant.
Definition of Plunge:
The plunge of the axis is the inclination of the axis below horizontal. 0° is horizontal, 90° is vertical.