The UK flag consists of three elements: the cross of St. George (red on white) for England, the cross of St. Andrew (white diagonal on blue) for Scotland, and the so-called cross of St. Patrick (red diagonal on white) for Ireland. The original Union Jack adopted in 1606 was symmetrical: the red cross of St. George outlined in white overlaid on top of a St. Andrew’s flag, which was blue with a white X.
In 1801, an Act of Union which made Ireland a co-equal member of the United Kingdom made it necessary to add a symbol for Ireland to the flag, but without obliterating any of the existing symbols. If the St. Patrick’s cross had been centered on the diagonal stripes, then St. Andrew’s cross would have been relegated to an inferior position, basically serving only as a border for St. Patrick’s. But Scotland was the senior of the two kingdoms, so this was unsatisfactory. The solution was to divide the diagonal stripes diagonally, so that the red St. Patrick’s cross would take up only half of each stripe, and so that half devoted to St. Andrew would take the place of honor. Thus, in the two hind quarters, the white St. Andrew’s cross occupies the upper position, and in the two forward quarters, the red St. Patrick’s cross occupies the upper position.
Source: https://flagspot.net/flags/gb.html#design
