the Union Flag

In 1603, James VI of Scotland inherited the English and Irish thrones, thereby uniting the crowns of England, Scotland and Ireland.
In 1606, a new flag to represent this regal union was specified in a royal decree, according to which the flag of England and Scotland would be joined together, forming the flag of Great Britain and first union flag.
The current and second Union Flag dates from 1801 with the Act of Union 1800, which merged the Kingdom of Ireland and the Kingdom of Great Britain to form the United Kingdom of Great Britain and Ireland.
The new design added a red saltire, the 'cross of Saint Patrick', for Ireland.
The three component crosses that make up the Union Flag are:

1. The red St. George's Cross, which represents England
2. The white diagonal St. Andrew's Cross, representing Scotland
3. The red diagonal St. Patrick's Cross, representing Ireland