A more thorough explanation:

Letters of Slains: Historical letters written to the Crown by the family members of a person who was killed, agreeing to the offender's request for a pardon from the king or queen. A pardon could not be granted without the family's agreement. These letters were also known as letters of slanes.

During medieval times, if someone committed a crime such as murder, they could apply for a pardon from the king or queen. However, the pardon could only be granted if the family of the person who was killed agreed to it. The family members would write letters to the Crown, known as letters of slains, stating that they forgave the offender and agreed to the pardon.

For example, if John killed William, John could apply for a pardon from the king or queen. If William's family agreed to the pardon, they would write letters of slains to the Crown, stating that they forgave John and agreed to the pardon. Only then could the pardon be granted.

Another example would be if a knight killed another knight in a duel. The family of the slain knight could write letters of slains to the Crown, agreeing to the pardon of the knight who killed their relative.

These examples illustrate how letters of slains were used in medieval times to ensure that justice was served while also allowing for forgiveness and mercy.