Note 1 to entry: In usual coordinates ${\mathrm{d}}^{4}x=\text{j}{c}_0\text{d}t\cdot \text{d}x\cdot \text{d}y\cdot \text{d}z$.

Note 2 to entry: The four-volume element is a Lorentz invariant.

Note 3 to entry: The coherent SI unit of four-volume element is metre to the power four, ${\text{m}}^4$.