The national flag of Spain (Spanish: Bandera de España),[a] as it is defined in the Constitution of 1978, consists of three horizontal lines: red, yellow and red, the yellow stripe being twice the height of each red stripe. Traditionally, the middle stripe color was called by the archaic term gualda (weld, a natural dye); hence the flag's nickname la Rojigualda (the red–weld). The middle stripe bears the coat of arms of Spain, being mandatory in several cases.

The origin of the current flag of Spain is the naval ensign of 1785, Pabellón de la Marina de Guerra, by Decrée of Charles III of Spain, where it is also referred as national flag. It was chosen by Charles III among 12 different flags designed by Antonio Valdés y Bazán. The flag remained marine-focused for most of the next 50 years and flew over coastal fortresses, marine barracks and other naval properties. During the Peninsular War, the bicolor flag was used by marine regiments fighting inland, and began to be also used in Army camps and raised by many Spaniards as a symbol of resistance. In 1843, during the reign of Queen Isabella II of Spain, the flag was adopted by all the Armed Forces.

From 18th century to nowadays, the color scheme of the flag remained intact, with the exception of the Second Republic period (1931–1939); the only changes affected to the coat of arms. Wikipedia