The United States Constitution took effect as the nation’s supreme law in 1789.
The Constitution was drafted at the Philadelphia Convention in 1787 and signed on September 17 of that year. It replaced the Articles of Confederation, the first constitution of the United States, after the required number of states ratified it.
The new federal government began operating under the Constitution on March 4, 1789. Congress organized its first sessions, and George Washington was inaugurated as the first president on April 30, 1789, in New York City.
The year 1787 is therefore the year of drafting and signing, while 1788 marks the ratification by the ninth state needed for the Constitution to take effect. The Bill of Rights was proposed in 1789 and ratified in 1791, so those dates describe later developments.