Ab diagram is one, either on a stereographic or equal area projection, in which planes are directly plotted as cyclographic traces. It is used for relatively less deformed terrains. For complicated terrains involving a superimposed deformation history, p diagram is preferred. b diagram is also used for simple unstraining technique to find the orientations of planes or lines in undeformed state.
The fig shown is for a completely cylindrical fold since there is a single point of intersection.