This rule represents the economic profit obtained with the control of certain stable trade routes (e.g., the routes of the Hanseatic League).
A trade route is composed by two end provinces (or posts) and one or more areas which the route goes through. The effect of the commercial route is determined during the income assignation.
Whenever a player controls a post in a trade route, he get an additional ducat, unless there is a unit in one of the areas that the route crosses (including the posts) that does not belong to any one of the players who control the route posts.
If a post is an end of more than one route, the player controlling the post gets 1 ducat for every route that is not blocked by the enemy.
If one of the posts is occupied by an autonomous garrison, the player controlling the other post still gets the money. However, if there is also an enemy unit, the route is blocked.