When you consult a financial website and see the CAC 40 quotation scrolling by, the code that appears in the data feeds is often PX1. This technical symbol, associated with the prefix “indexeuro,” simply designates the mnemonic identifier of the main French stock index on the Euronext Paris platform. Understanding what the indexeuro PX1 encompasses means grasping how the most followed barometer of the French economy operates.
Why the closing auction changes the reading of the PX1 index
The composition and calculation method of the indexeuro PX1 are well documented. However, a less visible phenomenon alters how this index is formed on a daily basis: the concentration of trading during the closing auction.
Between 2018 and 2025, the share of volume executed at the close for CAC 40 stocks increased from about one-third to more than half, according to the Autorité des marchés financiers (AMF). This shift is not trivial.
The closing price serves as a reference for calculating the value of the PX1 index. It also serves as the basis for ETFs replicating the CAC 40 and for derivatives indexed to it. When the majority of transactions are concentrated in the last minutes of the session, the final fixing takes on a disproportionate weight. Index fund managers, derivatives desks, and funds that rebalance their portfolios intervene massively at this precise moment.
For a retail investor, this means that the closing price of PX1 mainly reflects the activity of institutional investors. If you place an order in the middle of the day, the price you obtain may deviate significantly from the reference value published in the evening. The AMF specifies that this trend does not structurally degrade liquidity outside of extreme stress periods, but it transforms the dynamics of price formation.
To understand the indexeuro PX1 stock index in its daily reality, one must integrate this shift towards the closing auction, which makes it an index increasingly shaped by passive management and automated strategies.
Market capitalization weighted: what this changes concretely
The indexeuro PX1 includes the 40 largest continuously listed capitalizations on Euronext Paris. Their weight in the index depends on their floating market capitalization, that is, the value of shares actually available for trading on the market.
Have you ever noticed that a single day of sharp decline in LVMH or TotalEnergies causes the entire CAC 40 to drop? This is the direct consequence of the weighting. Some very large companies weigh as much as dozens of others combined in the calculation of the index.
This mechanism has a practical implication: when you invest in a CAC 40 ETF, you are not buying 40 companies in equal shares. You are taking a concentrated exposure to a handful of dominant sectors (luxury, energy, banking). An equally weighted index would yield a very different result over the same period.
Periodic revision of the composition
The Scientific Council of Euronext indices regularly reviews the list of the 40 selected stocks. A company can enter or exit the index if its floating capitalization changes. This rebalancing causes buying and selling movements from index funds, which must mechanically adjust their portfolios.
The entry of a stock into the CAC 40 generates an influx of automatic purchases, while an exit triggers sales. These movements are predictable, and some traders anticipate them to profit.
Indexeuro PX1 and ESG criteria: a variant to know
In recent years, Euronext has offered an ESG version of the CAC 40 index. This variant filters the 40 stocks by applying environmental, social, and governance criteria. It excludes companies involved in certain controversial activities (armaments, tobacco, thermal coal).
This distinction has consequences for investors using products indexed to PX1:
- An ETF replicating the classic CAC 40 (PX1) includes all stocks without ethical filtering, including sectors with a high carbon footprint
- An ESG CAC 40 ETF excludes some of these stocks and overweights those that receive the best extra-financial ratings
- The performance of the two versions can diverge over several months, depending on the sectors that are rising or falling
The code PX1 always designates the classic CAC 40 index, without ESG filter. If you invest through a product labeled “ESG,” check the ISIN code of the underlying asset to know exactly which index you are following.
European regulation on benchmark indices: what changes for PX1
The indexeuro PX1 is subject to the European regulation on benchmark indices (Benchmark Regulation). This text governs the calculation methodology, governance, and transparency of indices used as references by financial products.
The AMF recently updated its doctrine to take into account the revision of this regulation. Management companies offering funds or ETFs indexed to the CAC 40 must update their prospectuses, specifying in particular:
- The exact name of the index administrator (Euronext) and its registration with ESMA
- The replacement scenarios if the index were to be modified or deleted
- Any potential conflicts of interest between the index administrator and the issuers of financial products
For an individual, this regulation remains little visible on a daily basis. It guarantees that the PX1 index is calculated according to audited and transparent rules, which is not the case for all indices worldwide.
Impact on CAC 40 ETFs and index funds
Funds that replicate the indexeuro PX1 must now document more precisely their dependence on this index. In the event of a major methodological change from Euronext, the manager is required to inform the holders and, if necessary, propose a substitute index.
This regulatory framework enhances the reliability of PX1 as a market reference. It also reminds us that a stock index is not just a simple number, but a legally regulated product, whose governance has a direct impact on the savings products that are linked to it.
The indexeuro PX1 remains the most direct entry point to track the performance of large listed French companies. Its reading gains in precision when one integrates the growing weight of the closing auction, the sector concentration related to the weighting, and the European regulatory framework that guarantees its transparency.