in special relativity. The dot product of the basis vectors is the Minkowski metric, as opposed to the Kronecker delta as in Euclidean space. It is convenient to rewrite the definition in matrix form: A ⋅ ⋅ B = ( A 0 A 1 A 2 A 3 ) ( η η 00 η η 01 η η 02 η η 03 η η 10 η η 11 η η 12 η η 13 η η 20 η η 21 η η 22 η η 23 η η 30 η η 31 η η 32 η η 33 ) ( B 0 B 1 B 2 B 3 ) in which case η μν above is the entry in row μ and column ν of the Minkowski metric as a square matrix. The Minkowski metric is not a Euclidean metric, because it is indefinite (see metric signature). A number of other expressions can be used because the metric tensor can raise and lower the components of A or B. For contra/co-variant components of A and co/contra-variant components of B, we have: A ⋅ ⋅ B = A μ μ η η μ μ ν ν B ν ν = A ν ν B ν ν = A μ μ B μ μ so in the matrix notation: A ⋅ ⋅ B = ( A 0 A 1 A 2 A 3 ) ( B 0 B 1 B 2 B 3 ) = ( B 0 B 1 B 2 B 3 ) ( A 0 A 1 A 2 A 3 ) while for A and B each in covariant components: A ⋅ ⋅ B = A μ μ η η μ μ ν ν B ν ν with a similar matrix expression to the above.