A privy seal refers to the personal seal of a reigning monarch, used for the purpose of authenticating official government documents.
The Privy Seal of England can be traced back to the reign of King John. It has been suggested that it was originally the seal that accompanied the person of the Sovereign, while the Great Seal was required to remain in the Chancery. Eventually it became a requirement that almost no non-judicial document could pass under the Great Seal without a warrant from the Privy Seal. The Barons wrested control of the Privy Seal away from the King by 1312 and it was replaced by the signet as the King's personal seal. The Privy Seal became the heart of a second writing office and clearing house, with warrants being sent to the Chancery and Exchequer under orders made with the Signet. (By 1400 the Signet was in the custody of the King's Secretary, and as such it is the precursor to the seals of office held by today's Secretaries of State.)
The Great Seal Act 1884 effectively ended the use of the Privy Seal by providing that it was no longer necessary for any instrument to be passed under the Privy Seal.
The Privy Seal of England was originally overseen by the Clerks of the King's Chamber, but soon came to be kept by the controller of the wardrobe; by 1323, however, the distinct office of Keeper of the Privy Seal had emerged (the first Keeper, who was not also Controller, having been appointed in 1307). The present-day title of this office, Lord Privy Seal, is first recorded in 1539.
Prior to its adoption as the Privy Seal of the United Kingdom after the Act of Union 1707, its most notable appearance in Scottish history was its alleged capture by the Scots in the aftermath of the Battle of Old Byland in 1322, when the Scots led by Robert the Bruce almost captured Edward II, who was forced to flee, leaving behind many personal possessions including the Privy Seal.