Direito espontâneo: Law Merchant

Origins: The Law Merchant, or Lex Mercatoria, was originally a body of rules and principles laid down by merchants themselves to regulate their dealings. It consisted of usages and customs common to merchants and traders in Europe. (...) International commercial law today owes some of its fundamental principles to the Law Merchant as it was developed in the medieval ages. This includes choice of arbitration institutions, procedures, applicable law and arbitrators, and the goal to reflect customs, usage and good practice among the parties.
(...) The Law Merchant provided quick and effective justice. This was possible through informal proceedings, with liberal procedural rules. The Law Merchant rendered proportionate judgements over the merchants’ disputes, in light of “fair price”, good commerce, and equity.

Choice of judge: Judges were chosen according to their commercial background and practical knowledge. Their reputation rested upon their perceived expertise in merchant trade and their fair-mindedness. Gradually, a professional judiciary developed through the merchant judges. Their skills and reputation would however still rely upon practical knowledge of merchant practice. These characteristics serve as important measures in the appointment of international commercial arbitrators today

Legal concepts introduced by the Law Merchant: Less procedural formality meant speedier dispensation of justice, particularly when it came to documentation and proof. Out of practical need, the medieval Law Merchant originated the “writing obligatory”. By this, creditors could freely transfer the debts owed to them. The “writing obligatory” displaced the need for more complex forms of proof, as it was valid as a proof of debt, without further proof of; transfer of the debt; powers of attorney; or a formal bargain for sale. The Law Merchant also strengthened the concept of party autonomy: whatever the rules of the Law Merchant were, the parties were always free to choose whether to take a case to court, what evidence to submit and which law to apply." Wikipedia