The Spanish flag is a currently a red and yellow horizontal triband (the yellow stripe is in the middle, and is twice as tall as each red band). A simplified version of Spain’s coat of arms is pictured on the yellow band, towards the hoist side. The height of this flag is two-thirds the width. Spain’s flag was officially adopted on December 19, 1981.
The Spanish coat of arms depicts two crown-topped pillars of Hercules (with red banners displaying the motto in Latin, “PLUS ULTRA,” meaning “More Beyond,” alluding to Columbus’ discovery of the New World). The two columns flank a shield that displays a castle, a lion wearing a crown, red and yellow vertical stripes, chain mail, and three fleurs-de-lis in an oval in the center. An ornate red and golden crown sits atop the shield.
The Spanish pledge to the flag is called the Jura de Bandera (meaning “oath of the flag”).
But before that there were many variations!
For more in depth detail see Wiki link