This is part 3 of an open series of posts on terminology; see *part 1* and *part 2*.

As another way of indicating the dimensions of space and time, let *space-time* mean unified 3D space + 1D time, and let *time-space* mean unified 3D time + 1D space.

Consequently, *Galilean time-space* requires what I’m calling the *co-Galilean* transformation. And *Lorentz time-space* requires what I’m calling the *co-Lorentz* transformation.

The appropriate way of measuring the rate of movement in space-time is speed and velocity, since time is the independent variable. Pace is equivalent to the inverse of speed but their independent variables differ.

The appropriate way of measuring the rate of movement in time-space is pace and celerity, since space (length or distance) is the independent variable. Speed is equivalent to the inverse of pace but their independent variables differ.