The squared norm of a four-vector a = (a^0, a^1, a^2, a^3) = a^0 + a is given in the standard basis using the + - - signature as 〈a, a〉 = (a^0)^2 - a·a and using the - + ++ signature as 〈a, a〉 = a·a - (a^0)^2,

dot product | four-vector | light cone | lightlike | Lorentzian inner product | Lorentzian space | metric signature | Minkowski space | negative lightlike | negative timelike | norm | positive lightlike | positive timelike | spacelike | timelike