This lecture introduces the concept of anharmonic cross ratio as a way to move beyond simple ratios and explore intertwined quadrilaterals. It delves into the birth of projective geometry, focusing on the properties of pascal & brianchon and the link between projective geometry and perspective. The lecture also covers practical applications such as regulating the meeting of beams in space and the use of polygons circumscribed to a cone. Various tools and techniques related to projectics and stereotomy are discussed, providing a comprehensive overview of projective geometry concepts.