The document signed in 1215 that limited the power of the English monarch is known as the Magna Carta. This landmark agreement, sealed by King John, was a response to the barons' rebellion against his arbitrary rule and heavy taxation. The Magna Carta established key principles, including the idea that everyone, including the king, is subject to the law. It laid the groundwork for the development of constitutional law and individual rights, influencing future democratic systems. This document is often heralded as an early step towards establishing fundamental rights and the concept of a government accountable to its people.