Belgium, like many other European nations, was a collection of duchies. This is why the Duchy of Brabant served as the inspiration for the banner which, years later, would serve as the national flag.
The Belgian flag has unusual proportions compared to other flags as these are 13:15.
The origin of the current flag of Belgium is the result of two key moments in its history. First, in 1789, the Brabant revolution. At the end of it, the Belgians hoisted for the first time a flag which represented national interests.
This flag, very similar to the German flag, was formed of three horizontal bands respectively red, black and yellow, colors coming from the emblem of Brabant.
In 1830, during the second uprising, the Belgians hoisted a similar flag which kept the same colors but changed the order: from top to bottom, the horizontal bands were then respectively red, yellow, then black. This last uprising was against Holland and the result was satisfactory as Belgium became an independent kingdom in 1831.
On January 23 of that same year, a slight change was made in the orientation of the flag: it was turned 90° to the right, so that the stripes became vertical. The color order became, left to right: black, yellow, and red.
This change was a way to compete with the French standard, because France was a world benchmark for political progress. This is therefore the reason why the Belgians have decided to adopt, in a certain way, this symbolism, by changing the orientation of the stripes of their national flag. However, this emulation absolutely did not cause an abandonment of native characteristics. The Belgians kept the colors that represented them, those of the arms of the Duchy of Brabant.
It can be said that the new events ended up changing the meaning of the colors of the flag. The black ended up representing the sacrifice of the citizens who fought for independence, the yellow the wealth of the nation and the red the blood shed by the heroes of the independence saga.