$$ \LARGE \begin{bmatrix} \color{red}a & \color{red}b & \color{red}c\\\color{black}d &\color{black} e &\color{black}f\end{bmatrix}^t = \begin{bmatrix} \color{red}a & \color{black}d\\ \color{red}b &\color{blacj} e \\ \color{red}c &\color{black} f\end{bmatrix}$$

Got it !

Transposition d'une matrice = échange lignes et colonnes
Transposition d'une matrice carréée= symétrique par rapport à la diagonale

$\LARGE \begin{bmatrix}1 & 2\\3 & 4\end{bmatrix}^t = $	$\LARGE \begin{bmatrix}1 & 3\\2 & 4\end{bmatrix}$
$\LARGE \begin{bmatrix}1 & x & 3\\4 & y & 0\end{bmatrix}^t = $	$\LARGE \begin{bmatrix}1 & 4 \\x & y\\3 & 0\end{bmatrix}$
$\LARGE \begin{bmatrix}1 & \color{red}2 & \color{red}3\\\color{blue}4 & 5 & \color{red}6\\\color{blue}7 & \color{blue}8 & 9\end{bmatrix}^t = $	$\LARGE \begin{bmatrix}1 & \color{blue}4 & \color{blue}7\\\color{red}2 & 5 & \color{blue}8\\\color{red}3 & \color{red}6 & 9\end{bmatrix}$