Cross of Burgundy flag

The Cross of Burgundy (Spanish: Cruz de Borgoña, Cruz de San Andrés), a form of St. Andrew's cross, was first used in the 15th century as an emblem by the Valois Dukes of Burgundy, who ruled a large part of eastern France and the Low Countries as effectively an independent state. The Duchy of Burgundy was inherited by the House of Habsburg on the extinction of the Valois ducal line. The emblem was then assumed by the monarchs of Spain as a result of the Habsburgs bringing together, in the early 16th century, their Burgundian inheritance with the other extensive possessions they inherited throughout Europe and the Americas, including the crowns of Castile and Aragon.

The Spanish monarchs continued to use it in their own arms even after they lost their Burgundian lands. From 1506 to 1701 it was used by Spain as a naval ensign, and up to 1843 as the land battle flag, and still appears on regimental colours, badges, shoulder patches and company guidons. The emblem also continues to be used in a variety of contexts in a number of European countries and in the Americas, reflecting both the extent of Valois Burgundy and the former Habsburg territories.

The banner strictly speaking dates back to the early 15th century, when the supporters of the Duke of Burgundy adopted the badge to show allegiance in the Armagnac–Burgundian Civil War. It represents the cross on which Andrew the Apostle was crucified. The design is a red saltire resembling two crossed, roughly-pruned (knotted) branches, on a white field. In heraldic language, it may be blazoned argent, a saltire ragulée gules.

...
Wikipedia