The invariant ring of pair of matrices
Let us consider the action of the general linear group $\mathrm{GL}_n(\mathbb{C})$ on the direct product $\mathcal{M}_n^d$
of $d$ copies of $\mathcal{M}_n$ by simultaneous conjugation sending $(X_1,\ldots, X_d)$ to $(gX_1g^{-1},\ldots,gX_dg^{-1})$
for any $g\in \mathrm{GL}_n(\mathbb{C})$ . This induces an action of $\mathrm{GL}_n(\mathbb{C})$ on the algebra $\mathbb{C}[\mathcal{M}_n^d]$ of polynomial
functions on $\mathcal{M}_n^d$. The algebra of invariants under this action, $\mathbb{C}[\mathcal{M}_n^d]^{\mathrm{GL}_n}$, is an important