Hello, Before starting, I would like to apologize for any errors in the use of symbols. This is my first time . I am studying the wonderful book of Barton Zwiebach, "A First Course in StringTheory". In chapter 02, I am experiencing for the first time with the mathematics of special relativity (Minkowski Spacetime). My question is on the definition of invariant interval ds. By definition, the invariant interval is given by -ds=η[μν]dx[μ]dx[ν] I am not able to understand the minus sign on ds. Is there any relationship with the idea of positive-definite condition? Others books use only ds for the invariant interval. Is there any advantage in using this convention? Another question would be about the invariant interval -ds[2.]. The definition of the invariant interval is very similar to the definition of Riemannian metric (metric tensor) g[ij]. (a) invariant interval → -ds=η[μν]dx[μ]dx[ν] (b) Riemannian metric → g=∑g[ij]dx⊗dx[j] Is there any direct relationship? What is the difference between them? I sincerely thank any reply .