Question mark
In the early 13th century, when the growth of communities of scholars (universities) in Paris and other major cities led to an expansion and streamlining of the book-production trade, punctuation was rationalized by assigning Alcuin's stroke-over-dot specifically to interrogatives; by this time the stroke was more sharply curved and can easily be recognized as the modern question mark.
Gibbs paradox
The question marks (?) serve as a reminder that we may not assume that the first n particles (i.e. 1 though n) are in system-A while the other particles (n through N) are in system-B. (This is further discussed in the next section.)