The US Constitution is rigid, federal and written. 

The United States Constitution is considered rigid in the sense that it is difficult to amend; it requires a formal process of proposal and ratification. This is unlike the British Constitution, which is flexible, i.e. it can be amended through an ordinary legislative process. 

The United States operates under a federal system, meaning that the power is shared between the national government and the state governments. This distinguishes the U.S. from the United Kingdom, which has a unitary system, where the central government holds more power.

The United States Constitution is written, meaning it is formally codified in a single document. The British Constitution is largely unwritten in the sense that it is made up of statutes, conventions and legal practices rather than a single written document.