Short answers, pulled from the story.
The Constitution of the United States stands as the oldest active codified constitution in human history. It serves as a supreme law that defines how the state is governed and limits the power of rulers.
Excavations in modern-day Iraq by Ernest de Sarzec in 1877 revealed evidence of the earliest known code of justice issued by Sumerian king Urukagina of Lagash. This ancient document relieved taxes for widows and orphans while protecting the poor from usury practiced by the wealthy.
The Polish-Lithuanian Commonwealth passed its Constitution on the 3rd of May 1791 becoming Europe's second oldest after the American example. France adopted its Constitution of 1791 shortly thereafter introducing revolutionary changes to monarchy.
The standard model described by Baron de Montesquieu divides government power into three branches: executive, legislative, and judicial. Most constitutions explicitly separate these functions to prevent any single entity from accumulating absolute authority.
Only two sovereign nations maintain wholly uncodified constitutions today: New Zealand and the United Kingdom. San Marino holds the distinction of having the oldest written document still governing a sovereign nation dating back to 1600.