Alston Householder introduced the convention of labeling matrices with upper case Roman letters ($A \text{,}$ $B \text{,}$ etc.), vectors with lower case Roman letters ($a \text{,}$ $b \text{,}$ etc.), and scalars with lower case Greek letters ($\alpha \text{,}$ $\beta \text{,}$ etc.). When exposing columns or rows of a matrix, the columns of that matrix are usually labeled with the corresponding Roman lower case letter, and the the individual elements of a matrix or vector are usually labeled with "the corresponding Greek lower case letter," which we can capture with the triplets $\{ A, a, \alpha \} \text{,}$ $\{ B, b, \beta \} \text{,}$ etc.
where $\alpha$ and $\chi$ is the lower case Greek letters "alpha" and "chi," respectively. You will also notice that in this course we start indexing at zero. We mostly adopt this convention (exceptions include $i \text{,}$ $j \text{,}$ $p \text{,}$ $m \text{,}$ $n \text{,}$ and $k \text{,}$ which usually denote integer scalars.)