Saint Patrick's Saltire or Saint Patrick's Cross is a red saltire (X-shaped cross) on a white field, used to represent the island of Ireland or Saint Patrick, the patron saint of Ireland. In heraldic language, it may be blazoned Argent, a saltire gules. Saint Patrick's Flag is a flag composed of Saint Patrick's Saltire.
The red saltire's association with Saint Patrick dates from the 1780s, when the Order of Saint Patrick adopted it as an emblem. This was a British chivalric order established in 1783 by George III. There is some evidence that a similar saltire was occasionally used to represent Ireland before this. It is often suggested that it derives from the arms of the powerful Geraldine or FitzGerald dynasty. Most Irish nationalists reject its use to represent Ireland as a "British invention".
After its adoption by the Order of Saint Patrick, it began to be used by other institutions. When the 1800 Act of Union joined the Kingdom of Ireland with the Kingdom of Great Britain, the saltire was added to the British flag to form the Union Flag still used by the United Kingdom. The saltire has occasionally served unofficially to represent Northern Ireland and been considered less contentious than other flags flown there.
The most widespread use of St Patrick's Saltire today is in the Flag of the United Kingdom. With the 1800 Act of Union that merged the kingdoms of Great Britain and Ireland, the red saltire was incorporated into the Flag of the United Kingdom as representing Ireland. The red saltire is counterchanged with the saltire of St Andrew, such that the white always follows the red clockwise. The arrangement accounts for the discontinuous look of the red diagonal lines, and has introduced a requirement to display the flag "the right way up", with the white line of St Andrew above the red of St Patrick in the upper lefthand quarter next to the flagpole. As with the red cross, so too the red saltire is separated by a white fimbriation from the blue field. This fimbriation is repeated for symmetry on the white portion of the saltire, which thereby appears wider than the red portion. The fimbriation of the cross of St George separates its red from the red of the saltire.