`  :::::::(responding to Tweedledee) A full decision tree enumerating all events that might happen is presented in the Grinstead and Snell book (an elementary probability textbook in the article's references, available online).  Although presented there as a decision tree, this tree is for all intents and purposes the same thing as a sample space.  A similar tree (from both Carlton and Chun, also in the references) showing only the events following ``player initially picks door 1`` is presented in the article in the ``Conditional probability by direct calculation`` section.  If you're having trouble understanding the text in this section or simply think it's unclear, it's fair to ask here for the text to be clarified.  I hope you don't take offense, but it seems like you're having trouble understanding the basic concepts of sample spaces and how they change in response to events.  This sort of discussion belongs on the /Arguments subpage as this article is not a tutorial on basic probability theory.  And, perhaps a more direct response, the article does not mention sample spaces because that's not how most references approach the problem.     `