As you already know, every variable has a name. Fortran 90 has rules about the names that you can give to variables. Fortran is restrictive programming languages in terms of variable names. The rules are:
What about these names: alpha, X+Y, HuNdReDs, Result, and x12?
Fortran actually uses these same rules for all of the names in a program. For example, the name that you specify on the program line has to follow the same rules about length, letters and digits, and so on. Notice that the program in the file rbtjnts.f is named rbtjnts, and that name follows all the rules.