Identifying the first place i and j are used is a start, but it doesn't really answer the question. The question is "why?", not "where did it start?" Of the e

Identifying the first place i and j are used is a start, but it doesn't really answer the question. The question is "why?", not "where did it start?" Of the explanations on the SO page, the fact that i is the first letter of index seems like the most likely. j is popular because it follows i.