[Zope] Stupid Newbie question

I am sure that the answer to this question has been staring at me ever since I started using Zope. Anyway, how do I find a list of 'builtin' variables such as 'id' ,'title', and 'title_or_id' etc.? I just realized that I had been using them without really understanding where they are defined. Could it be dangerous if I inadvertantly reused these names? Many thanks, Dave.