If I have already used $i,j,k$ as indices, and need two more, where should I look? $l,m$?, $m$? I know from my previous questions that I can use anything I want as long as I define it, but I use $n$ quite a bit, and it seems that using $m$ as an index could be confusing.
I have thought of using $s$ and $t$, or $s_i$ and $t_i$ which are the initials of the categorical variables over which I am indexing? I don't think that there would be a confusion between the use of $s$ in $\beta{_\text{style}}_s$ and the standard use of $s$ for standard deviation... but perhaps there is a more standard convention that will be more clear / intuitive?