Examples of four-vectors include the coordinates (*ct*, *x*, *y*, *z*) themselves, the four-current (*c*ρ, **J**) formed from charge density ρ and current density **J**, the electromagnetic four-potential (φ, **A**) formed from the scalar potential φ and vector potential **A**, and the four-momentum (*E*/*c*, **p**) formed from the (relativistic) energy *E* and momentum **p**. The speed of light (*c*) is often used to ensure that the first coordinate (*time-like*, labeled by index 0) has the same units as the following three coordinates (*space-like*, labeled by indices 1,..,3).

The scalar product between four-vectors *a* and *b* is defined as follows:

The laws of physics are also postulated to be invariant under Lorentz transformations. An object in an inertial reference frame will perceive the universe as if the universe were Lorentz-transformed so that the perceiving object is stationary.

See also: four-velocity, four-acceleration, four-momentum, four-force.