Projective geometry is a type of geometry that deals with the study of geometric shapes and their properties. It is different from Euclidean geometry, which is the study of geometric shapes and their properties in a flat plane. In projective geometry, the basic building blocks are points, lines, and planes, and these objects are considered to be "undefined" in the sense that they are not assigned any specific size or location. Instead, their properties are determined by the way they relate to one another, and this relationship is captured using a set of axioms and theorems. One of the main features of projective geometry is the concept of "duality," which states that any theorem that holds in projective geometry also holds if we interchange the roles of points and lines. This allows us to prove theorems in projective geometry by working with either points or lines, depending on which is more convenient.