The reason 'i' was the first letter available was because by default identifiers starting with, I think, I-N were integers instead of float.

When I first started, with BASIC, my variable of choice was T simply because it was easy to type NEXT T in the FOR loops (why not R? I dunno...T just stuck).

Other way around. I–N were integers rather than float because mathematics convention uses i, j, … as indices and n, m as sizes or counts.