Two opposite velocities — or lenticities — are invariant over time and space.

The standard Galileian transformation in the space-time domain is

Velocity *u* transforms as

Velocity is not invariant relative to a single inertial observation, but it is relative to observations with opposite relative velocities:

That is

Harmonic velocities are opposites and so are Galileian invariant in space-time.

Similarly, the standard Galileian transformation in the time-space domain is

Lenticity *n* transforms as

Lenticity is not invariant relative to a single inertial observation, but it is relative to observations with opposite relative lenticities:

That is

Harmonic lenticities are opposites and so are Galileian invariant in time-space. Notably, the pace of light reflected a given distance and back is invariant.

The time-space domain may be represented to a limited extent with the reciprocals of the Galileian transformation:

Velocity *u* transforms over distance as

Velocity is invariant relative to observations with opposite relative velocities:

That is