As noted *here*, there are two kinds of mean rates: the *time mean* and the *space mean*. If the denominators have a common time interval, the time mean is the arithmetic mean and the space mean is the harmonic mean. If the denominators have a common space interval (stance), the space mean is the arithmetic mean and the time mean is the harmonic mean.

For example, light reflected back from a mirror at known distance forms two successive trips whose mean rate is the space mean pace. Several measurements with the same apparatus have a time mean pace. The mean speed is the inverse of the mean pace, which is parallel to, but not the same as, the speed.

The general principle is that quantities with independent time (such as velocity) and a common time interval use ordinary (series) algebra but such quantities with a common space interval use *reciprocal arithmetic*. Alternately, quantities with independent space (stance) such as lenticity and a common space interval use ordinary algebra but such quantities with a common time interval use reciprocal arithmetic.

In other words, quantities over the same interval with independent variables use ordinary (arithmetic) algebra but quantities with different independent variables use reciprocal arithmetic to exchange them.

For example, addition of velocities with a common time interval use ordinary vector addition (e.g., **u** + **v**) but addition of velocities with a common space interval use reciprocal vector addition (e.g., ((1/**u**) + (1/**v**))^{−1} ≡ ((**u**+**v**)/**u·v**)^{−1} with **u**, **v**, **u·v**, **u**+**v** ≠ **0**.

The relativity parameter *γ* is based on a (3+1) spatial frame. The parameter *γ* in a temporal frame with a common time interval (*k* ≡ 1/*c* and **ℓ** ≡ 1/**v**) is:

*γ*² = (1 − **v·v**/c²)^{−1} ⇒ (1 − **ℓ·ℓ**/*k*²)^{−1}.

The parameter γ in a temporal frame with a common space interval (stance) is:

*γ*² = (1 − **v·v**/c²)^{−1} ⇒ H(1 − **ℓ·ℓ**/*k*²)^{−1} = ((1 − *k*²/**ℓ·ℓ**)^{−1})^{−1} = (1 − *k*²/**ℓ·ℓ**) ≡ (1 − **v·v**/*c*²) = 1/*γ*².