Thanks for the detailed answer. I now see more of the overall purpose of the macros in stringize.hpp, but your 2nd example does not in fact require the ## operator to get the behavior you want with Microsoft C++. If you delete the ## out of macro B, leaving everything else in place, there is no difference in the behavior; you still get p + q. I tried this with Microsoft Visual C++ 5 (v11) and Microsoft Visual C++ 2005 (v14): #define IM p, q #define A(im) B((im)) #define B(par) C par // no ## here #define C(x, y) x + y A(IM) // p + q So despite your impressive knowledge of Microsoft's preprocessor bugs, I'm not really convinced the ## is needed in stringize.hpp, at least with Microsoft C++. I see the similar ## usage in cat.hpp and iif.hpp, but that code seems limited to Metrowerks. I'm really only interested in Microsoft C++. In stringize.hpp was the only place ## was used to produce an invalid token when I built boost with Microsoft C++ ("bjam -sTOOLS=vc-8_0"). There may be others places, but those wheels didn't squeak for me.