We should also decide which way is up. I like the OpenGL coordinate
system, which has X increasing to the right, Y up, and Z coming out of
your monitor.
Non-symmetric objects that move should be defined so that increasing
their X position makes them look like they are moving forward. For
example, if modelling a car, the headlights should shine down the +X
axis and if you look out the sun roof you should be looking in the +Y
direction.