The flag of Britain - the United Kingdom and Northern Ireland - is officially called the Union Flag, because it embodies the emblems of the three countries under one Sovereign.
The emblems that appear on the Union Flag are the crosses of the three patron saints:

• the red cross of St George, for England, on a white ground
• the white diagonal cross or saltire of St Andrew, for Scotland, on a blue ground
• the red diagonal cross attributed to St Patrick, for Ireland, on a white ground
  
  
  

  The first Union Flag, in 1606, combined the national flag of England - the cross of St George - and that of Scotland - the cross of St Andrew. Wales is not represented in the Union Flag because when the first version of the flag appeared Wales was already united with England.
  

  The cross of St George	The cross of St Andrew	The cross of St Patrick

  
  The final version of the Union Flag appeared in 1801, following the union of Great Britain with Ireland, with the inclusion of the cross of St Patrick.