The number of Dyck paths in a square is well-known to equal the catalan numbers:
http://mathworld.wolfram.com/DyckPath.html
But what if, instead of a square, we ask the same question with a rectangle? If one of its sides is a multiple of the other, then again there is a nice formula for the number of paths below the diagonal, but is there a nice formula in general? I am also interested in asymptotics.