Here's a postcard showing many flags of Finland (also known as The Land of Postcards, especially on Postcrossing) ;)
The flag of Finland (Finnish: Suomen lippu, Swedish: Finlands flagga), also called Siniristilippu ("Blue Cross Flag"), dates from the beginning of the 20th century. It features a blue Nordic cross on a white background. Blue represents lakes and the sky, and white represents snow and the white nights of the Finnish summer. The state flag has a coat of arms in the centre, but is otherwise identical to the civil flag. The swallow-tailed state flag is used by the military. The presidential standard is identical to the swallow-tailed state flag but also has in its upper left corner the Cross of Liberty after the Order of the Cross of Liberty, which has the President of Finland as its Grand Master. Like Sweden's, Finland's national flag is based on the Scandinavian cross. It was adopted after independence from Russia, when many patriotic Finns wanted a special flag for their country, but its design dates back to the 19th century. The blue coloring is said to represent the country's thousands of lakes and the sky, with white for the snow that covers the land in winter. This color combination has also been used over the centuries in various Finnish provincial, military, and town flags.
As a coincidence, the flag of Finland is the same as the flag of Shetland with the colours reversed.