Poynings' Law is a constitutional principle in the United Kingdom that requires the certification of acts of the legislative branches by the monarch. The law states that any act of the legislature must be signed and dated by the monarch before it can become law. This ensures that the monarch has the power to review and approve legislation, and prevents the legislature from passing laws without the monarch's consent. The law has been in place since the 15th century and remains an important part of the UK's constitutional framework.