In 1844, the proposals of a joint committee were enacted for both countries by King Oscar I. A union mark was created, combining the flag colours of both countries, equally distributed. It was placed in the canton of each flag, including the merchant flags, which had until then been without any symbols of the union. The two countries received separate, but parallel flag systems, clearly manifesting their equality.
The union mark on its own was used as the naval jack in both countries, and as the flag of the common diplomatic representations of both countries abroad. The diplomatic flag had the proportions 4:5 of the union mark as it appeared in Swedish flags, unlike the square shape of the Norwegian version. It was also used on the pilot jack which was similar to the naval jack but had a white border.
The blue in the union mark would be the same as in the rest of the flag, usually the dark blue of the Norwegian flag. Swedish flags before 1905 also had a darker shade of blue than present flags.
The union mark was at first popular in Norway as a sign of Norway's equal status in the union. In Sweden, it was always seen by some people as a desecration of their flag, and one of its adversaries called it the "herring salad" (Norwegian: sildesalaten, Swedish: sillsalladen) because of its resemblance to a popular dish on the Scandinavian smörgåsbord. It came to be popularly known under this name in both countries and to this day is its common denomination in these languages.