On the Rhine River boats pass port to port, or as we would say on the one whistle side. That is the rule and they navigate accordingly, BUT if they need to pass stbd. to stbd. (two whistle side) all Rhine River boats have a blue board about 1 meter square on the right side of the wheelhouse. Normal this board is in the horizontal position or laying flat. If the pilot wants to pass on the starboard side, he rotates the board to the vertical position so its visible to the pilot on the approaching boat. No whistles are blown. The pilot on the approaching boat rotates his board to show that he understands the passing arrangements. At night the board has a light in the center of it which is lit. I believe they refer to this as "passing board to board".