Dreptunghiul: paralelogramul cu unghi drept (900).

                                                                                                                         

                                                                                                      -lungime

                                                                                                                                                  -latime

                                        

                                                    Aria:

                                  

                                                   Perimetrul : 

                                  

                                   Teoreme:

                                               - daca un patrulater are toate unghiurile congruente (drepte),el este un dreptunghi sau un patrat.

                                               - diagonalele unui dreptunghi sunt congruente.