In relativity theory, it's a common habit to use a quadruplet x=(x_0, x_1, x_2, x_3) with x_0=ict (or with c=1, x_0=it ) instead of (t,x_1,x_2,x_3)\in \mathbb R^4, and to use the formal Euclidean metric \|x\|^2=\sum_{i=0}^3x_i^2 instead of the Minkowski pseudo-metric -t^2 + \sum_{i=1}^3x_i^2...