Note 1 to entry: The vectors of an orthonormal base are usually denoted by $e_1\text{,}{e}_2\text{,}\mathrm{...}\text{,}{e}_n$; for a three-dimensional Cartesian coordinate system they are often denoted by $e_x\text{,}{e}_y\text{,}{e}_z$ or $i\text{,}j\text{,}k$.