Square, cube, octahedron, equations

We know that $|x| + |y| - 1 = 0$ is the equation of a square having its vertices on the axes. I asked to represent the equation $|x| + |y| + |z| - 1 - 0$, believing to obtain a cube in space. But I obtain an octahedron. Why? And how do you get a cube? # with SageMath 7.3 var('x, y, z') f = abs(x) + abs(y) + abs(z) - 1 implicit_plot3d(f, (x, -1, 1), (y, -1, 1), (z, -1, 1), color='aquamarine ')