The United Kingdom’s Union Flag combines the crosses of Saint George, Saint Andrew and Saint Patrick.
The red upright cross of Saint George represents England. The white diagonal saltire of Saint Andrew represents Scotland, and the red diagonal saltire of Saint Patrick represents Ireland. Their layered arrangement creates the design commonly called the Union Jack.
The first version was created in 1606 after the crowns of England and Scotland were united under one monarch. The present design dates from 1801, when the Act of Union joined Great Britain and Ireland. It retained the cross associated with Saint Patrick, even though most of Ireland later became independent.
A common mistake is to call every version the Union Jack. “Union Flag” is the formal term in many contexts, while “Union Jack” originally referred especially to a flag flown at sea. The design also appears in the flags of several countries historically connected to Britain, including Australia and New Zealand.