Process of adoption

The national flag is often, but not always, mentioned or described in a country's constitution, but its detailed description may be delegated to a flag law passed by the legislative, or even secondary legislation or in monarchies a decree.
Thus, the national flag is mentioned briefly in the Basic Law for the Federal Republic of Germany of 1949 "the federal flag is black-red-gold" (art. 22.2 Die Bundesflagge ist schwarz-rot-gold), but its proportions were regulated in a document passed by the government in the following year. The Flag of the United States is not defined in the constitution but rather in a separate Flag Resolution passed in 1777.
Minor design changes of national flags are often passed on a legislative or executive level, while substantial changes have constitutional character. The design of the flag of Serbia omitting the communist star of the flag of Yugoslavia was a decision made in the 1992 Serbian constitutional referendum, but the adoption of a coat of arms within the flag was based on a government "recommendation" in 2003, adopted legislatively in 2009 and again subject to a minor design change in 2010. The Flag of the United States underwent numerous changes because the number of stars represents the number of states, proactively defined in a Flag Act of 1818 to the effect that "on the admission of every new state into the Union, one star be added to the union of the flag"; it was changed for the last time in 1960 with the accession of Hawaii.
A change in national flag is often due to a change of regime, especially following a civil war or revolution. In such cases, the military origins of the national flag and its connection to political ideology (form of government, monarchy vs. republic vs. theocracy, etc.) remains visible. In such cases national flags acquire the status of a political symbol.
The flag of Germany, for instance, was a tricolour of black-white-red under the German Empire, inherited from the North German Confederation (1866). The Weimar Republic that followed adopted a black-red-gold tricolour. Nazi Germany went back to black-white-red in 1933, and black-red-gold was reinstituted by the two successor states, West Germany and East Germany following World War II. Similarly the flag of Libya introduced with the creation of the Kingdom of Libya in 1951 was abandoned in 1969 with the coup d'état led by Muammar Gaddafi. It was used again by National Transitional Council and by anti-Gaddafi forces during the Libyan Civil War in 2011 and officially adopted by the Libyan interim Constitutional Declaration.