$X= \begin{bmatrix} \boldsymbol x_1 \\ \boldsymbol x_2 \\ \vdots \\ \boldsymbol x_m \end{bmatrix} = \begin{bmatrix} x_{11} & x_{12} & x_{13} & \dots & x_{1d} \\ x_{21} & x_{22} & x_{23} & \dots & x_{2d} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ x_{m1} & x_{m2} & x_{m3} & \dots & x_{md} \end{bmatrix}$