A standard or orthonormal basis for Minkowski space is a set of four mutually orthogonal vectors { e 0, e 1, e 2, e 3 } such that − − η η ( e 0 , e 0 ) = η η ( e 1 , e 1 ) = η η ( e 2 , e 2 ) = η η ( e 3 , e 3 ) = 1 and for which η η ( e μ μ , e ν ν ) = 0 when μ μ ≠ ≠ ν ν .