The flag of Great Britain, often referred to as the King's Colour, Union Flag, Union Jack, and British flag (retroactively prefixed as being the "first" such flag, in order to distinguish it from the modern flag of the United Kingdom), was used at sea from 1606 then more generally from 1707 until 1801 as the flag of the Kingdom of Great Britain, and is the precursor to the modern Union Jack.
Set down in 1606 by Royal decree of James VI and I, the flag's form combined the flag of the Kingdom of England and flag of the Kingdom of Scotland and was specifically intended for maritime use by vessels of both kingdoms, which since 1603 had shared a single monarch in a personal union following the Union of the Crowns. In 1634, Charles I proclaimed that the flag's use be restricted to vessels of the Navy Royal and Royal Scots Navy, stating that it "be reserved as an ornament proper for Our own Ships and Ships in our immediate Service and Pay, and none other".