Stock market indices such as the NIFTY 50 or NIFTY Bank are not permanent, unchanging lists of companies. They are maintained by index providers — in India, primarily NSE Indices Limited — who review the composition of each index on a scheduled basis, typically every quarter or every six months. This review process is called reconstitution, and the resulting adjustments to the weights or members within the index are broadly referred to as rebalancing. The two terms are related but distinct: reconstitution refers to adding or removing companies from an index, while rebalancing refers to adjusting the proportion, or weight, that each existing member holds within the index.

An index is not a fixed list of companies — it is periodically reviewed and updated to reflect changes in the market it is designed to represent.

Inclusion in a major index is determined by objective criteria, which vary by index but generally include factors such as market capitalisation, trading liquidity, the proportion of shares available for public trading (known as free float), and the company's listing history. When a company grows large enough and liquid enough to meet these criteria, it may be added to an index. Conversely, if a company's market capitalisation falls significantly or its trading volumes dry up, it may be removed. Because large funds — particularly passive or index funds — are mandated to hold all the constituents of their benchmark index in the correct proportions, any change in index composition triggers real buying and selling activity in those stocks by fund managers seeking to match the new index. This is why index reconstitution announcements can sometimes be followed by noticeable price movement in the stocks being added or removed, as market participants anticipate the flow of funds.

Today's data offers a small illustration of how index-level dynamics play out in practice. The NIFTY IT index closed down 0.87%, while the NIFTY Bank index rose 0.59% — two indices that draw from very different pools of companies and are governed by their own weighting rules. Sector-level indices like these are also periodically rebalanced to ensure that no single stock dominates the index beyond a prescribed cap, often set at 33% or similar thresholds depending on the index methodology. Understanding this process helps investors who hold index funds or ETFs appreciate that what looks like a passive investment still involves a structured, rule-based mechanism working in the background to keep the portfolio aligned with its benchmark. The weights, members, and rebalancing schedule for any index are publicly available in the index methodology documents published by NSE Indices, which are a useful resource for those who want to understand what exactly they own when they invest in an index fund.