Hugot gave us an overview of the vexillological considerations found in the constitutions of the world. His paper was entitled A Journey Through Constitutional Vexillology.
Interestingly many constitutions make no mention of the national flag. Often it is that older nations already have a prescribed flag and it is over looked. However younger nations often have clear descriptions of the flag, a few even provide a graphic.
Hugot pointed out that in the US, only first abandoned constitution, the Articles of Confederation, mentions the US flag.