X and Y are independent uniform r.vs in the common interval (0,1). Determine where Z = X + Y.
There are 2 ranges of z: $0 < z < 1$ and $1<z<2$.
The diagrams given for both ranges to find $F_z(z)$ is given as follows.
For $1<z<2$,
$$F_z(z) = 1-P(Z>z) = 1-\int_{y=z-1}^{1}\int_{x=z-y}^{1} 1 \,dx \,dy$$
I understand the integral over x, but I don't understand how to get the integral over y, i.e. $\int_{y=z-1}^{1}$. How is the $y=z-1$ obtained?