We move on from triangles to quadrilaterals. The guiding idea is simple: a parallelogram can be seen as “two congruent triangles glued together” along a diagonal, and from this idea all its properties follow. The section presents the definition of a quadrilateral (convex and non-convex), the trapezium with its area, the hierarchy that classifies all quadrilaterals, and finally the parallelogram, the rhombus and the rectangle with their respective characteristic conditions and some classic proofs.