Fulcrum Lines in RPD

A fulcrum line is an imaginary line around which an RPD will tend to rotate. 
Fulcrum lines may be in the horizontal, frontal or vertical plane. 
An important point to remember: Indirect retainers provide resistance to rotational movement of RPD away from the denture bearing tissues around the retentive fulcrum line. 

LOCATION OF STABILIZING FULCRUM LINES
KENNEDY CLASS I - the fulcrum line passes through the rest areas on the most posterior abutment on either side of the arch.

KENNEDY CLASS II - the fulcrum line passes diagonally through the most posterior occlusal rests. In the image, the line is passing through abutment on distal extension side and the most posterior abutment on opposite side.

KENNEDY CLASS III - the fulcrum line is non-existent.A tooth supported RPD is totally supported by occlusal rests and has no rotation because of no soft tissue movement.

In above image,
posterior tooth on right side which has a poor prognosis and will eventually be lost,
fulcrum line is considered the same as though posterior tooth were not present.

In above image, with nonsupporting anterior teeth, adjacent edentulous area is considered to be tissue- supported end, with diagonal fulcrum line passing through two principal abutments as in class II arch

KENNEDY CLASS IV - the fulcrum line passes through the two most anterior rests adjacent to the edentulous space.