In the Galilean transformation, there are two separate invariants that two inertial frame of reference always agree on between two separate events:

- time
- length, given by the Pythagorean theorem

However, in special relativity, neither of those are invariant separately, since space and time are mixed up together.

Instead, there is a new unified invariant: the spacetime-interval, given by:

$cΔt_{2}−(Δx_{2}+Δy_{2}+Δz_{2})$

Note that this distance can be zero for two events separated.