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D-branes, as I currently understand them, are submanifolds of spacetime on which open strings can end with Dirichlet boundary conditions. On the other hand, type II string theory is a theory of oriented closed strings - the difference between IIA and IIB coming from the choice of GSO projection. Based on these observations, I have the following questions:

  1. If type II string theory is built from closed strings, why are D-branes considered in the theory?
  2. If open strings are included in type II, does the GSO projection act on the open strings? If so, how?
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  1. Type II string theory also contains open strings. The statement on Wikipedia is misleading.
  2. The GSO projection removes part of the open string spectrum. Most notably, it removes tachyonic modes from the theory, making it stable. You can read this up in detail in Polchinski, Volume II, Chapter 10.
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