The aim of this paper is to focus on some aspects of the formation of merchants' manuals and tariffs, in order to stress, 1) the multiplicity of sources used by the compilers, 2) the usefulness of the comparison among texts bearing the same kind of information in order to follow, as far as possible, the circulation of the knowledge. Two case studies will be presented here: the first is an excerpt coming from the ‘tariff of Syria' (16th C.) held by the St Mark’s Library in Venice, compared to two contemporary sources; the second exemple is a broader comparison among ten ‘conoscimenti di merci’ (descriptions of goods) coming from ten merchants' manuals.