The Vikings flew flags on their ships but these did not have much resemblance to the presentday national flags of the Nordic countries. The Vikings picked up the habit from the Arabs who got the idea from the Chinese. For the Chinese the colors of their silk flags represented philosophical and religious concepts, while the Arabs and Vikings used the colors to represent dynasties and individual chieftains.
The general design of the Nordic flags was born when medieval knights started using banners with crosses on their third crusade to the Holy Land at the end of the 12th century. The early flags were square, just like the Swiss flag, but then they were extended from Sweden’s 10/16 to the Åland Islands 10/19 proportions.
According to a legend King Erik of Sweden left on a crusade to convert the Finns to Christianity in 1157. Before a decisive battle, as Erik prayed to God for a sign, he and his troops saw a golden cross in the zenith of the sun against the blue sky. Erik subsequently adopted the yellow cross against a blue background as his banner and won the battle, later becoming known as "Erik the Saint"
The first time the Swedish flag is used as a flag is during the reign of Gustav Vasa. When his son Erik XIV is courting Mary Stuart he gives specific instructions to his admirals to carry the Swedish flag on their trip to Scotland. When he himself sails to Finland on August 2, 1569 the ship is decked out with Swedish flags in the form of streamers. His brother Johan notes in a letter "that the golden or yellow cross has been used in the crest of Sweden for ages and shall therefore be used as flags, banners and standards."
During the reign of Gustav II Adolf when Sweden was at the height of its might, the ship of the king always flew a two-edged Swedish flag. Until then it was exclusively the symbol of the King, but in the first Swedish flag law dated November 6, 1663, all Swedish trading ships were authorized to fly the yellow and blue. The "private" flag was two-edged while the "state" flag that flew from naval ships and fortifications was three-edged. To distinguish a ship carrying the king a white square with the national coat of arms was added to the three-edged flag. Other members of the royal family could fly flags with the minor coat of arms, distinctions that are still in use.
Oscar II popularized the use of the Swe-dish flag by erecting a flag pole on the Stockholm castle. The flag was flown whenever the King was in town, a tradition that lives on today. Oscar II’s love for the flag initially was met with considerable skepticism but eventually most national institutions started flying the flag and eventually the general public also adopted the pretty tradition. The growing nationalism around the turn of the century was a great promoter of a Swedish symbol and the custom to fly the flag on auspicious occasions became more and more widespread. The problem today is that so many rightist and racist groups have adopted the flag as their symbol that it has become almost suspect. In the late 60s, on the other hand, the flag was often defamed at antinationalistic protests. Artist Carl Johan de Geer was taken to court when he defiled the flag at an exhibition during the height of anti-nationalistic feel-ings in 1967. But generally a very liberal use of the flag is accepted in Sweden.