The precise form of the constitutional documents depends upon the type of entity, such as corporations or private associations.
In civil law jurisdictions, the company's constitution is normally consolidated into a single document, often called the charter.
It is quite common for members of a company to supplement the corporate constitution with additional arrangements, such as shareholders' agreements, whereby they agree to exercise their membership rights in a certain way.
Conceptually a shareholders' agreement fulfills many of the same functions as the corporate constitution, but because it is a contract, it will not normally bind new members of the company unless they accede to it somehow.
In most legal systems unincorporated associations are not required to have formal written constitutions, but many of the larger and more complex organisations would be almost impossible to administer without one.