How to untensor a tensor?

What should I do to convert a matrix of matrices to a non-nested matrix?

$$\left( \begin{array}{cc} \left( \begin{array}{cc} {a_1} & {b_1} \\ {c_1} & {d_1} \\ \end{array} \right) & \left( \begin{array}{cc} {a_2} & {b_2} \\ {c_2} & {d_2} \\ \end{array} \right) \\ \left( \begin{array}{cc} {a_3} & {b_3} \\ {c_3} & {d_3} \\ \end{array} \right) & \left( \begin{array}{cc} {a_4} & {b_4} \\ {c_4} & {d_4} \\ \end{array} \right) \\ \end{array} \right)\to \left( \begin{array}{cccc} {a_1} & {b_1} & {a_2} & {b_2} \\ {c_1} & {d_1} & {c_2} & {d_2} \\ {a_3} & {b_3} & {a_4} & {b_4} \\ {c_3} & {d_3} & {c_4} & {d_4} \\ \end{array} \right)$$

• ArrayFlatten Jun 30, 2022 at 8:31
• We got an excellent answer pointer, this would be a good opportunity for new members looking to gain some reputation to write up an answer with some nice examples. Jun 30, 2022 at 9:06
• mathematica.stackexchange.com/questions/119/… Jun 30, 2022 at 19:54

To expand on the comment, this is exactly what ArrayFlatten is designed to do. Straight from the documentation:
ArrayFlatten[{{$$m_{11}$$,$$m_{12}$$,...},{$$m_{21}$$,$$m_{22}$$,...},...}] creates a single flattened matrix from a matrix of matrices $$m_{ij}$$
sub[i_] = Partition[Table[Alphabet[][[j]][i], {j, 4}], 2];