Can anyone explain to be Cini's paradox ?

There is an 8 X 8 grid (square shape). It is cut into two equal tziums and two equal right angled triangles. Strangely it is arranged as a 13 X 8 rectangle. 8 X 8 grid has 64 squares. But 13 X 5 rectangular grid has 65 squares. Where is that 1 extra square coming from ?