I'm solving problem 3.D in H. Georgi *Lie Algebra etc* for fun where one is to compute the matrix elements of the direct product $\sigma_2\otimes\eta_1$ where $[\sigma_2]_{ij}\text{ and }[\eta_1]_{xy}$ are two different Pauli matrices in two different two dimensional spaces. Defining the basis in our four dimensional tensor product space $$\tag{1}\left|1\right\rangle = \left|i=1\right\rangle\left|x=1\right\rangle\\ \left|2\right\rangle = \left|i=1\right\rangle\left|x=2\right\rangle\\ \left|3\right\rangle = \left|i=2\right\rangle\left|x=1\right\rangle\\ \left|4\right\rangle = \left|i=2\right\rangle\left|x=2\right\rangle$$ Now we know that when we multiply representations, the generators add in the sense of $$\tag{2}[J_a^{1\otimes2}(g)]_{jyix} = [J_a^1]_{ji}\delta_{yx} +\delta_{ji}[J_a^2]_{yx}, $$ where the $J$s are the generators corresponding to the different representations $D_1$ and $D_2$ ($g$ stands for the group elements). Using all of this I find that in the basis of $(1)$ the matrix representation of the tensor product is given by $$\tag{3}\sigma_2\otimes\eta_1 = \begin{pmatrix} 0 & 1 & -i & 0 \\ 1 & 0 & 0 & -i \\ i & 0 & 0 & 1 \\ 0 & i & 1 & 0 \end{pmatrix}$$ I am not asking you to redo the calculations$^*$ for me but does $(3)$ make sense? Appendix: The Pauli matrices $$\begin{align} \sigma_1 &= \begin{pmatrix} 0&1\\ 1&0 \end{pmatrix} \\ \sigma_2 &= \begin{pmatrix} 0&-i\\ i&0 \end{pmatrix} \\ \sigma_3 &= \begin{pmatrix} 1&0\\ 0&-1 \end{pmatrix} \,. \end{align} $$ ___ $*$ Or perhaps you know the answer already.