Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In projective geometry, a dual curve of a given plane curve is a duality partner of a given curve. The dual of a projective plane is naturally identified with the space of lines in the projective plane. Thus if C is a curve in a projective plane, then the dual curve is a curve in the dual projective plane consisting of the set of lines tangent to C. There is a map from a curve to its dual, sending each point to the point dual to its tangent line.