In ancient as well as modern markets, the presence of rules and authorities is an indispensable constant enabling economic transactions. Usually, such regulations are first created on a local level. But the facilitation, strengthening and improvement of growing trade relations on an interregional, European and a global scale lend urgency to the need for common rules (even if they encounter opposition on the local or regional level). Contemporary examples for that necessity include various interventions in the European sphere and beyond, aimed at harmonizing market rules up to the various projects of unification of European law in the past decades.We have testimony that such a necessity already started to be felt from the antique times on, in both the Greek and Roman world. Within that context, the decree regarding weights and measures (IG II2 1013) dating from the end of the second to the beginning of the first century B.C., is of particular importance. This psephisma contains a series of provisions aimed at the regulation of commercial relations in Athens regarding the quantification of traded goods by both volume and weight. The manufacture and use of weights and measures is ordered; the competences and punitive powers towards private individuals and magistrates are identified by the various bodies in charge; the metrological reforms of the choinix and the commercial mine are introduced, resulting in a metrological standardization and—as we will see—harmonization. On one hand, these legislative interventions demonstrate the importance, recognized by the authorities, of a comprehensive regulation of all the subjects involved in measuring goods and to all its essential aspects, to ensure fairness and trust in the markets of Athens, Piraeus, and Eleusis. On the other hand, through the reforms of the choinix and the commercial mine, the decree aims at a metrological harmonization with the Roman system, enabling easy convertibility and thereby facilitating commerce not only among Rome and Athens themselves, but among all territories familiar with either notation throughout the Mediterranean. By leaving a mere local dimension, the legislative effort of one city aims at gaining usefulness on an interregional scale.