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 P: 38 Hello! In the study of electric and magnetic fields, two equations are called the constitutive relations of the medium (the vacuum, for example): $\mathbf{D} = \mathbf{\epsilon} \cdot \mathbf{E}\\ \mathbf{B} = \mathbf{\mu} \cdot \mathbf{H}$ But in a generic medium (non linear, non isotropic, non homogeneous) $\mathbf{\epsilon}$ and $\mathbf{\mu}$ are tensors. Now, why not matrices with dimension 3x3? $\mathbf{E}$ and $\mathbf{H}$ are "simple" three-dimensional vectors. I know that a matrix is a particular case of a tensor, but so why do we never use the term "matrix" in this context? A matrix could exist only if a particolar system of coordinates is defined, whereas a tensor can always exist: is it the reason for calling $\mathbf{\epsilon}$ and $\mathbf{\mu}$ tensors and not just matrices? Thank you anyway! Emily