Bending of rocks due to plastic deformation is expressed in nature by folds. Every fold contains an imaginary axial plane which divides the fold as symmetric as possible in two different halves, dividing the fold into two  limbs. The fold axis is  the line formed where the axial plane intersects folded rock layer as shown in the line a-b.

Folds having both limbs dipping towards the fold axis are called synclines, while folds of which limbs are dipping away from the fold axis are called anticlines.