This is the region bounded by the planes x + y + z = 1,
x = 0, y = 0, and z = 0.

This is the region bounded by the planes x + y + z = 1,
x = 0, y = x, and z = 0.

This is the region bounded by the planes x + z = 1,
y + z = 1, x = 0, y = 0, and z = 0.

This is the region bounded by the planes y + z = 1,
x = 0, y = x, and z = 0.