As someone who many years ago programmed in Fortran, $i, $j, $k for loop indicies.

For those of you too young to remember such things, variables in Fortran (we are talking Fortran IV - not recent things corrupted by quiche-eating Pascal programmers) were never declared. Integer variables (by default) started with the letters I through to O (no case dependancy, either).

Oh, the joys of FORMAT statements with nested implied do loops.