Tetradas, short for: Tonnage estimates of trade through the Danish Sound, is a series of statistics based on the Danish Sound Toll Registers Online for the period from 1670 to 1856. The Danish Sound Toll Registers are registers of the taxes paid to the Danish King by ships passing through the narrow strait between the North Sea and the Baltic. The earliest registers date back to 1497; from 1574 onwards they constitute an almost uninterrupted series until the abolition of the Danish Sound dues in 1857. In the project Sound Toll Registers Online (STRO) the registers were digitized and transcribed into a relational database with more than four million entries.

Tetradas builds on this work. In the Tetradas-Project, the entries for the years 1670 to 1856 were homogenized and standardised and the wide variety of pre-modern weights and measures was converted to metric equivalents with the goal of estimating the tonnage of shipping through the Danish Sound.

The results of the Tetradas-Project are available online. They will be developed further in the coming years. The following developments are planned or in progress:

• Conversion of pre-modern weights and measures will be expanded to the period 1497 to 1669.
• Visual analysis of distribution networks
• Implementation of record linkage methods for identifying shipmasters and their sailing patterns