It all started back in 1603, when James VI of Scotland became James I of England, Wales and Scotland, uniting the three countries.
He blended the St George’s flag of red and white cross with the Scottish St Andrew’s flag, which was white saltire and blue and created the Union Flag as we know it today.
King James insisted that this flag should be flown on the bowsprit of all of his ships, so the Royal Navy ships commenced to fly the new Union Flag in 1606.
As the King signed off letters and orders by using the Latin version of his name ‘Jacobus Rex’ and personel letters as Jac, the new mast on the ships became known as the Jackstaff.
Also, the flag should only be known as the Union Jack when it is being flown on a naval ship. If it is shown on a building or flagpole it should be called the Union Flag.