The scope of this paper is to point out the importance of trade usages as a source of autonomous commercial law whose origins date back to the Middle Ages (lex mercatoria). At the beginning, the elements (subjective / objective) that are necessary for the custom to acquire a legal character are presented. For a complete analysis of the trade usages, it is necessary to compare them with other autonomous sources of law, in particular with the customs identified as codified commercial customs. The issue of the application of trade usage is also regulated in international codifications with regard to the subjective and / or objective concept. The paper compares the application of trade usages under the Civil Obligations Act of 1978, which advocates a subjective concept of application in contrast to the Civil Obligations Act of 2006, which adopts modern and international legal solutions. When it comes to proving trade usages in court, it is necessary to interpret the principle of iura novit curia. In conclusion author analyses a hierarchical scale of sources of commercial law according to Civil Obligations Act of 1978 and Civil Obligations Act of 2006 as well as the application of trade usages according to dispositive and ius cogens provisions.