The usual models of Constitutions are: Written or Unwritten; Rigid or Flexible; and Federal or Unitary.
Written or Unwritten
Constitutions may be written like that of the U.S or unwritten and based on conventions like the British. The Indian Constitution is written, but conventions also play provided that they are in conformity with the provisions of the Constitution.
Rigid or Flexible
Constitutions may be rigid or flexible. Rigid constitutions are those constitutions where the amending procedure is very difficult. On the other hand, amendment procedure is easy in the case of flexible constitutions. Federal constitutions are usually rigid. Indian Constitution is a mixture of rigid and flexible. Certain provisions of the constitutions (e.g.: articles 2, 3, 4 and 169) can be amended by simple majority in the houses of parliament. Other provisions can be amended under article 368 by the Houses of Parliament by a special majority of 2/3rd members present and voting and a majority of the total membership in each house. However during 60 years there were as many 94 amendments. This disproves the charge of the rigidity of the Indian Constitution.
Federal or Unitary
Constitutions may also be federal or unitary. The Constitution of the U.S is federal where as that of the U.K is unitary. In a unitary constitution, all powers are vested in the central government. The subordinates units function like the agents of the central government. In a federal constitution the powers are divided between the central government and the state government; both exercising powers in their respective spheres in their own rights and independently. An independent Supreme Court arbitrates any disputes between the Union and the States. Indian Constitution cannot be fitted into either federal or unitary model. It has features of the both. It cannot be considered unitary because it provides for distribution of executive and legislative powers between the Union and the States. It cannot be considered strictly federal because the residuary powers vest in the Union. Indian polity is a dual polity but with a single citizenship. It could be both unitary and federal according to requirements of time and circumstances. Under article 249, the Union Parliament can invade the State List. Under articles 356 and 357, on the ground of failure of constitutional machinery in any state, all its executive and legislative powers may be taken over by the Union. Under articles 352 to 354, the constitution can be converted into an entirely unitary one inasmuch as during proclamation of emergency.