The colors started to be used to symbolize Belgium during the end of the 18th century, in defiance to the Austrian rule. The colors are taken from those used by the constituent regions of Belgium, corresponding to the red lion of Hainaut, Limburg and Luxembourg, the yellow lion of Brabant, and the black lion of Flanders and Namur. The order of the colors on the flag was not obvious though, and initially some other versions were used, e.g. the flag of the Brabant Revolution (1789-1790):
and in the Belgian revolution 1830:
The official version has the unusual ratio 13:15, although flags with 2:3 ratio are also common. On the royal palace, a flag with the even more unusual ratio 4:3 is flown, in order to look more proportional from below:
Noteworthy is also the naval ensign of Belgium, which looks drastically different than the national flag:
