I am really confused as to when I am supposed to use just "de" and when I am supposed to use de with le or la to form "du" and "de la". Right now I can't figure out if there is a rule or if it is just a matter of personal preference.
I have a picture dictionary that I use for my vocabulary training and some terms use just de and others use du or de la. For example (and this is just one of many instances I've come across in my dictionary) The Great Barrier Reef is called (at least in my picture dictionary) "La Grande Barrière de Corail" (notice that in this one only "de" is used), while The Tropic of Capricorn is called "Le tropique du Capricorne" (notice that here "du" is used). Why couldn't both have used "de" or for that matter "du"? Is there a rule here that I am missing or is this just arbitrary and simply a matter of preference meaning that both could be used depending on what we like better?
In particular I am not asking for different usages of partitive article. Specifically I am interested in why certain terms use de and why others use de + le (to form du) or de la. I am familiar with the uses of partitive article de and the rules of when it should be used, but my dictionary contains terms which are composed of multiple words where those rules don't necessarily apply (or at least I can't seem to fit those terms into rules for the use of de, de la or du) as in the examples I laid out above. I would like to know if there is a rule for terms like that when de, du or de la should be used. Another example would be le siège du chauffeuf (driver's seat) and le levier de vitesses (gear lever). The rules for partitive articles don't seem to have anything to do with these terms (at least I can't see the relationship, but if there is one I'd really appreciate the explanation). L'affichage du volume (gas meter display - on a gas pump) is another example where I don't understand why it couldn't also be l'affichage de volume. I am looking for a rule (if there is one) as to how these terms are formed using de or du/de la.