Descending weight structure

For an index to be allowed as the underlying for futures and options, SEBI does not just look at how many stocks it has. It also checks how the weights are arranged.

A descending weight structure means the constituents are ranked so weight falls as you go down the list — the biggest constituent has the highest weight, and no smaller-ranked stock is ever given a bigger weight than one ranked above it.

Per SEBI's circular dated 30 October 2025 (Chapter 2), a non-benchmark index must meet four prudential norms before derivatives can be introduced on it:

1. Minimum of 14 constituents.
2. Top constituent weight ≤ 20%.
3. Combined weight of the top 3 constituents ≤ 45%.
4. Descending weight structure.

Where an existing index does not meet these on the day of review, exchanges must bring it into line on a set timeline: BANKEX (BSE) and FINNIFTY (NSE) by 31 December 2025, in a single tranche; BANKNIFTY (NSE) by 31 March 2026, in four monthly tranches. In a tranche adjustment, the excess weight above the desired target is divided by the number of tranches remaining — for example, a constituent at 28% against a 20% target has its excess of 8% divided across 4 tranches, cutting the weight by 2 percentage points a month.

An index has five constituents ranked by weight: 18%, 16%, 14%, 12%, 8% (with the rest spread across smaller stocks). Every rank has a strictly lower weight than the one above it, so this index shows a proper descending weight structure, and its top constituent at 18% also clears the 20% cap.

Now suppose the third-ranked stock rallies and its weight rises to 17%, overtaking the second-ranked stock, whose weight has meanwhile drifted down to 15%. The order by rank is no longer strictly falling, so the index now fails the descending-weight-structure test — even though no single constituent breaches the 20% cap and the top three still total well under 45%. The four SEBI norms are checked separately; passing three of them does not excuse failing the fourth.

Chapter 2, section 2.5, lists descending weight structure as one of four SEBI prudential norms (October 2025 circular) for derivatives on non-benchmark indices, alongside the 14-constituent minimum, the 20% top-constituent cap and the 45% top-3 cap, and gives the BANKEX/FINNIFTY/BANKNIFTY implementation timeline.

• Descending weight structure is one of four separate conditions, not a summary of the other three — a question can ask for all four together or isolate this one.
• Top constituent ≤ 20%; top 3 combined ≤ 45% are two different caps, easily transposed by mistake.
• The implementation timeline differs by index: BANKEX and FINNIFTY, single tranche by 31 Dec 2025; BANKNIFTY, four monthly tranches by 31 Mar 2026.