Trade alliances are the Large scale political actors in Sothis. Members of a trade alliance charge merchants from other members no tarrifs, giving them an advantage. Tarrifs are typically charged on incoming goods, not outgoing ones, so it is cheaper to buy foriegn goods from a merchant within ones own alliance, provided the merchant went to buy them at the place they were produced. City states typically form trade alliances with enough neighbors to form a choke point, so that the distance to sail from one end to the other is too great to be worth making non stop. This means controlling the flow of foriegn goods through the alliances territory, and having the opportunity to tax these goods, as well as collecting docking fees, etc. Trade alliances shift somewhat, as rivalries and internal competition as well as changes in leadership among members will influence a particular city states allegiance.