A tensor is not just a matrix — it's a geometric object whose components transform
precisely under coordinate changes. T^i_j transforms with one Λ and one Λ⁻¹.
The metric tensor gᵢⱼ converts between upper (contravariant) and lower (covariant) indices: Vᵢ = gᵢⱼ V^j.
Einstein summation: repeated upper/lower index means sum over all values.