Note 1 to entry: The position four-vector denotes an event in space-time.

Note 2 to entry: The squared four-magnitude of a position four-vector

$\underset{\_}{\underset{\_}{x}}:=(x_0,x_1,x_2,x_3)=(\text{j}{c}_{0}t,x,y,z)$ is $S^2(\underset{\_}{\underset{\_}{x}},\underset{\_}{\underset{\_}{x}})=-c_0^2{t}^2+x^2+y^2+z^2$

Note 3 to entry: The coherent SI unit of the position four-vector is metre, m.