Subscripts to a variable; why a letter shoud come before a digit?

I noticed that I cannot create a variable $f_{2s}$ since Mathematica seems to leave a space between the subscripts 2 and s implying that $f_{2s}$ is not recognised in later calculations. But I see that I can create the variable $f_{s2}$ instead without any problem. Is there a way to create the variable $f_{2s}$ in Mathematica?

Thank you.

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Maybe Subscript[f, 2, s] will fits your requirements. It induces a comma between 2 and s like in two variable sequences. – tchronis Dec 10 '13 at 7:47
There are many issues with work with subsecripts but leaving them: Subscript[f, Row[{2, s}]] select it and evaluate in place via menu or Ctrl+Shift+Enter. – Kuba Dec 10 '13 at 9:47
What about for the subscript type 2 "escape" "comma" "escape" and then s? – Chrissy Dec 11 '13 at 12:00