Final answer:
The feudal contract was the set of unwritten rules between a lord and his vassal, while the Magna Carta was a significant written document that established due process and trial by jury in England.
Step-by-step explanation:
The set of unwritten rules that determined the relationship between a lord and his vassal was called the feudal contract. However, an important written document related to the feudal era that codified certain principles was the Magna Carta, signed in 1215. This landmark document established the relationship between the king and the nobles in England, limiting royal power and affirming the principles of due process and trial by jury, which became a precedent influencing Western legal systems, including that of the United States.