Prolog check precedence of letters

I am new to Prolog and guys do you have any idea on writing a procedure checking precedence of small letters like:

precedes(a, b) --> Yes
precedes(c, d) --> Yes
precedes(m, w) --> Yes
precedes(q, a) --> No

But not using the following procedure?
precedes(X, Y) :-
     X @< Y.