A single die is rolled and all countries cross index that number on a chart with determines a base number from one to six, depending on the country. In some scenarios, certain countries actually double the amount of variable income received. Ducats can also be saved from turn to turn, and can be traded or loaned to other players. Now that you have collected your income for the turn, the question then becomes, what to do with it. The first obvious answer is to build and maintain military units.
All military units cost three ducats to build or maintain.
You can disband units at this time by simply not maintaining them. Units must be built in home nation cities, and only one unit may be build in each city per year. Another use for ducats is bribery. By spending a sufficient number of ducats, you can eliminate an opponent's unit, or even convert it into one of your own. The owning player can attempt to block you by counterbribing the unit, however.