Rigid Constitution is a constitution embodied in a special and distinct enactment, the terms of which cannot be altered by ordinary forms of legislation. The U.S. Constitution, which cannot be changed without the consent of three-fourths of the state legislatures or through a constitutional convention, is of this type.

A flexible constitution is one that is easily influenced by change and is readily amended by general legislation.

Merits of Flexible Constitution:

i. It prevents internal revolts and revolutions: In a flexible constitution the nation is protected against internal revolts and revolutions. For instance, there have been fewer revolts and revolutions in England because of the flexible constitution of that country. The life of the people because of the flexible constitutions of that country.

The life of the people has not been disturbed there quite often, while in France, which is very close to England, monarchy has been abolished and so far five constitutions have been enacted because of revolutions and odd circumstances.

ii. Adjustment: Adaptability or elasticity is the chief merit of a flexible constitution. It enables the people to keep pace with changes in society. The British Constitution, for instance, has been able to undergo imperceptible alterations, apart from deliberate amendments in recent years. There has been a tremendous change in the powers of the monarch.

Demerits of Flexible Constitution:

i. Instability: A flexible constitution is said to be in a state of perpetual flex and it is less stable than a rigid constitution because it can be easily amended. It is liable to fall a prey to the whims and caprices of political demagogues who are moved by passions and emotions rather than by reason.

ii. Instrument in the hands of the Judges: A flexible constitution becomes a plaything of judicial courts, because they interpret it according to their own will and give it a meaning of their own liking and choice.

A constitution is a set of fundamental principles or established precedents according to which a state or other organization is governed.

A constitutional convention is an informal and uncodified procedural agreement that is followed by the institutions of a state. In some states, notably those Commonwealth of Nations states that follow the Westminster system and whose political systems derive from British constitutional law, most government functions are guided by constitutional convention rather than by a formal written constitution. In these states, actual distribution of power may be markedly different from those the formal constitutional documents describes. In particular, the formal constitution often confers wide discretionary powers on the head of state that, in practice, are used only on the advice of the head of government.

The classification of Constitutions: 

i. Rigid Constitution: A rigid constitution is one which cannot be amended, in the manner in which ordinary laws are passed amended or repealed. If a special procedure or organ is needed for its amendment, it is a rigid constitution. Ordinary legislature of the country is not competent to amend it in the ordinary legislative procedure. Under a rigid constitution distinction is always maintained between a constitutional law and an ordinary law, since a constitutional law is regarded as superior to an ordinary law.

The US Constitution has been very rigid constitution. It has got only 27 amendments during the past 220 years of its life. Ordinary law in U.S.A. can be passed by a simple majority of the Congress, whereas the constitutional laws can be amended only by the agreement of two-thirds majority of the Congress and three-fourths of the states.

ii. Flexible Constitution: A flexible constitution is one which can be amended in an ordinary legislative process by the ordinary legislature. A Constitutional law and an ordinary law are treated alike. They are placed on an equal footing. All constitutional amendments can be made by a simple majority of the legislature.

The constitution of England is a typical example of a flexible constitution. The British Parliament is competent to pass, amend or repeal any constitutional law in an ordinary legislative process as both constitutional laws and ordinary laws are treated alike.

iii. Indian Constitution strikes a balance between the rigidity and flexibility. The Constitution of India is neither wholly rigid nor wholly flexible. It is partly rigid and partly flexible. It is because of the fact that for the purpose of amendment, our constitution has been divided into three parts:

(a) Certain provisions of the constitution can be amended by a simple majority in Parliament. (Flexible)

(b) Certain provisions can be amended by a two-third majority in Parliament and ratified by at least fifty percent of the states. (Rigid)

(c) The remaining provisions can be amended by Parliament by two-third majority. (Rigid)

Essential Qualities of a Good Constitution are mentioned below:

(i) Clarity or Definiteness: It means that every clause of the constitution should be written such a simple language, as should express its meaning clearly.

(ii) Brevity: The constitution should not be lengthy. It should contain only important things and unimportant things should be left out.

(iii) Comprehensiveness: It means that the constitution should be applicable to the whole country or besides the central government, there should be mention of the structure and powers of state or provincial governments. Mention should also be made about the important matters relating to the rights and duties of the government and the citizens.

(iv) Flexibility: The constitution should not be too rigid to hinder the process of amendment when needed.

(v) Declaration of rights: A good constitution must contain the fundamental rights of the people. In the constitutions of countries like India, Soviet Union, China, France, America, Japan and Italy such types of declarations have been made.

(vi) Independence of Judiciary: Independence of Judiciary is another quality of a good constitution. The judiciary should not be under the control of the executive and it should function freely and act as the guardian of the Fundamental Rights of the people without favour or fear. 

According to Gilchrist, Constitution of a state is that body of rules or laws, written or unwritten which determine the organisation of government, the distribution of powers to the various organs of government and the general principles on which these powers are to be exercised. Whereas for Woolsey, Constitution is the collection of principles according to which the powers of the government, the rights of the governed and the relations between the two are adjusted.

The kinds of Constitutions:

i. Written Constitution: A written constitution means a constitution written in the form of a book or a series of documents combined in the form of a book. It is a consciously framed and enacted constitution. It is formulated and adopted by a constituent assembly or a council or a legislature.

ii. Unwritten Constitution: An unwritten constitution is one which is neither drafted nor enacted by a Constituent Assembly and nor even written in the form of a book. It is found in several historical charters, laws and conventions. It is a product of slow and gradual evolution. The government is organised and it functions in accordance with several well settled, but not wholly written rules and conventions.

iii. Flexible Constitution: A Flexible Constitution is one which can be easily amended. Several political scientists advocate the view that a flexible constitution is one in which the constitutional law can be amended in the same way as an ordinary law. Constitutional amendments are passed in the same manner by which an ordinary law is passed.

iv. Rigid Constitution: The Rigid Constitution is one which cannot be easily amended. Its method of amendment is difficult. For amending it, the legislature has to pass an amendment bill by a specific, usually big, majority of 2/3rd or 3/4th. For passing or amending an ordinary law, the legislature usually passes the law by a simple majority of its members.

v. Evolved Constitution: An evolved constitution is one which is not made at any time by any assembly of persons or an institution. It is the result of slow and gradual process of evolution. Its rules and principles draw binding force from the fact of their being recognised as ancient, historical, time-tested and respected customs and conventions.

vi. Enacted Constitution: An Enacted Constitution is a man-made constitution. It is made, enacted and adopted by an assembly or council called a Constituent Assembly or Constitutional Council. It is duly passed after a thorough discussion over its objectives, principles and provisions. It is written in the form of a book or as a series of documents and in a systematic and formal manner.