Most states sport a constitution which codifies the powers and rights that recognized entities are entitled to. Often, a constitution also circumscribes limits to those powers and rights. The recognized entities are typically branches of government, constituent territories (such as states within a federal nation), and sometimes the citizenry as a collective institution, with the constitution granting them inalienable rights.
Britain is unusual in lacking a document that serves as a written constitution. (The Catholic Church also lacks a constitution.) With its long history of relative political stability, UK governance is instead guided by its accumulation of legal conventions, statutes, treaties, and judicial decisions.
In stark contrast, Germany’s Basic Law is a response to the turmoil of the 20th century, especially the Nazi era. Its detailed constitution serves as much as an elaborate check on political power as it does a grant.
No constitution, even Germany’s, states all the rules covering the distribution of power. Instead, a constitution lays out a basic structure and mechanics.
The difference between the constitutional informality of the UK and the specificity of Germany’s constitution reflects national political evolution. Whereas older constitutions are brief, newer ones are more complicated. This partly reflects the growth of state power in all spheres of life. Newer constitutions spell out that power so as to preserve it for the entitled entities.
Above all, in setting rights and powers, constitutions are intended to establish rule of law. This is a guise. Constitutions commonly only suggest the values and mores upon which laws are supposed to be drafted, and by which states are supposed to conduct themselves.
The US constitution in its preamble indicates nothing more than the intent to “establish justice, ensure domestic tranquility, provide for the common defence, promote the general welfare, and secure the blessings of liberty.”
Disparity grows in time between the formal rules of a constitution and exercised political power. In reading the US constitution, one would think its electoral college a powerful body, as they are elected to meet and select a president. In practice, the electoral college has become a toothless ratification of citizen votes on a state-by-state basis.