Under Article 145(3), "any case involving a substantial question of law as to the interpretation of the Constitution" must be decided by a Bench of at least five judges. Such a Bench is called a Constitution Bench.

Constitution bench is the name given to the benches of the Supreme Court of India which consist of at least five judges of the court which sit to decide any case "involving a substantial question of law as to the interpretation" of the Constitution of India or "for the purpose of hearing any reference" made by the President of India under Article 143. This provision has been mandated by Article 145 (3) of the Constitution of India. The Chief Justice of India has the power to constitute a Constitution Bench and refer cases to it.

However, in several cases, constitutional issues have been decided by smaller Benches as well.

The scope of Article 145(3) was decided in Rao Shiva Bahadur Singh v. State of Vidhya Pradesh, (1955) 2 SCR 446 which held that only substantial questions involving interpretation of the Constitution must be heard by a Constitution Bench, and other ancillary questions can be referred back to a smaller Bench to decide. The first matter to be heard by a Constitution Bench was A.K. Gopalan v. State of Madras which dealt with the interpretation of Articles 21 and 22.

A matter can be referred to a larger Bench only by judicial order. But a 13-judge Bench to reconsider the famous Kesavananda Bharati judgment was initially constituted without a judicial order of reference and had to be dissolved after two days of the tense hearing.

The Constitution of India does not mandate seven or nine-judge Benches. Article 145 says that to decide a 'substantial question of law related to the interpretation of the Constitution' or requiring the Supreme Court's advice under Article 143, where the President seeks clarification on a matter, a Bench must have a minimum of five judges.

When the case at hand has many constitutional questions that even a five-judge Bench cannot answer adequately, a reference to a larger Bench becomes crucial. The interpretation of the Constitution by these Benches sets a precedent for not just other Courts in the country but also smaller Benches of the Supreme Court itself.

Constitutional benches of the Supreme Court of India are typically constituted in specific circumstances when important questions of constitutional law need to be decided. These benches are made up of a larger number of judges and are tasked with interpreting and clarifying provisions of the Indian Constitution. Constitutional benches are formed in the following situations:

Interpretation of the Constitution: In the Supreme Court of India, a constitutional bench is constituted when there is a substantial and significant question of constitutional law or a need to interpret specific provisions of the Indian Constitution.

Conflict of Prior Judgments: This occurs in situations where authoritative guidance is required on a matter of constitutional significance, and also when there is a conflict or inconsistency between the decisions of different benches of the Supreme Court, or a need to reconsider or overrule a prior judgment.

Reference by a Smaller Bench: Additionally, a smaller bench may refer a matter to a larger constitutional bench if it involves substantial questions of constitutional law that require a decision by a higher number of judges.

Presidential Reference: The President of India, under Article 143 of the Constitution, may also refer a matter to the Supreme Court for its opinion, especially when such references involve significant constitutional questions.

Public Interest Litigation (PIL): Lastly, in cases involving Public Interest Litigation (PIL) or matters of substantial public interest with constitutional implications, the court may choose to refer the matter to a constitutional bench.

The composition of a constitutional bench varies in terms of the number of judges, typically consisting of a larger panel of five or more judges. The decisions made by constitutional benches hold great legal and constitutional significance as they are binding and often reshape the Indian constitutional landscape by establishing precedents and guiding future interpretations.

The Chief Justice of India or senior judges have the discretion to form a constitutional bench. This is important for the growth and interpretation of India's constitutional law.

The Supreme Court usually hears most cases with a 2 or 3 judge division bench. However, a Constitutional bench is an exception.

A major judgment by the Supreme Court was the Kesavananda Bharati judgment on April 24, 1973. Sri Kesavananda Bharati of a Hindu religious mutt in Kerala filed this. He challenged the 24th, 25th, and 29th Amendments to the Indian Constitution on grounds of unconstitutionality.

This case was heard by 13 judges, making it one of the largest benches in Indian legal history. Chief Justice S. M. Sikri, Justice J.M. Shelat, Justice K.S. Hegde, Justice A.N. Grover, Justices A.N. Ray, Justice P. Jaganmohan Reddy, Justice D.G. Palekar, Justice H.R. Khanna, Justice K.K. Mathew, Justice M.H. Beg, Justice S.N. Dwivedi, Justice A.K. Mukherjee and Justice Y.V. Chandrachud were part of this bench.

The case was assigned to this bench as it revolved around significant constitutional questions about Parliament's amendment powers. The bench took half a year to hear it and deliver the final judgment.

In a 7:6 decision, the Supreme Court endorsed the Constitution's basic structure doctrine. It stated that democracy, secularism, federalism, and rule of law, fundamental features of the Constitution, cannot be amended by parliament. The court also asserted that the judiciary's review power is a critical part of the Constitution that Parliament can't alter.

The Chief Justice of India has the official power to put together this bench and assign cases to it.

References:

1. https://www.scobserver.in/journal/where-are-the-seven-and-nine-judge-constitution-benches/
2. https://byjus.com/ias-questions/what-is-a-constitutional-bench/
3. https://www.scobserver.in/journal/number-of-constitution-bench-decisions/
4. https://www.aspireias.com/rajya-sabha-tv/Constitution-Bench
5. https://www.indialegallive.com/is-that-legal-news/what-is-a-constitution-bench/
6. https://judgments.ecourts.gov.in/KBJ/?p=home/intro