# Corporate action

*https://stockmarketstack.com/glossary/corporate-action · next to Stock Market Data APIs*

**Definition:** An event that changes a security's shares, price basis or identity: a split, a dividend, a spin-off, a merger, a ticker or name change, a reverse split before a delisting. Price history has to be adjusted for most of them or it stops being comparable across the event, and the adjustment convention is the vendor's choice rather than a fact.

## How it works

A corporate action is anything an issuer does that changes the shares outstanding, the basis of the
price, or the identity of the security. The common ones are splits and reverse splits, cash and
stock dividends, spin-offs, rights issues, mergers and acquisitions, and ticker or name changes.

The mechanics run through a set of dates rather than a single one. The issuer declares the action
and a record date; ex-date conventions then determine who is entitled — FINRA Rule 11140 sets how
the ex-dividend date is established for transactions in the over-the-counter market. Announcing a
record date too late is itself regulated: 17 CFR 240.10b-17 makes untimely announcement of record
dates a prohibited act. Downstream, the events are distributed as reference data: Nasdaq publishes a
Daily List of dividends, distributions and splits, FINRA publishes its own for OTC securities, and
DTCC carries the whole lifecycle between institutions as ISO 20022 messages.

For anybody working with prices, all of that reduces to one problem. A four-for-one split quarters
the price without anything happening to the company, so an unadjusted series reads as a 75 per cent
crash on that date. Any high, low, close, return, moving average or volatility figure that spans the
event is wrong unless the series has been adjusted — and volume has to be adjusted the other way.

## The adjustment is a convention, not a fact

This is the part that surprises people. Splits are arithmetic and every vendor handles them the same
way. Dividends are not: adjusting for them requires deciding whether the cash is reinvested, at what
price, whether special distributions and return-of-capital count, how the factors compound, and how
much of the history gets restated when a new one arrives. Every provider makes those choices, few
document all of them, and the results differ — which is why the same ticker's adjusted close on the
same day is not the same number at two vendors.

Spin-offs and mergers are worse, because the security itself changes. A spin-off splits one price
history into two, and whether the parent's history is adjusted for the value that left is a
judgement. A ticker change breaks a series unless the vendor carries a permanent identifier
underneath the symbol, which is the problem
[identifiers exist to solve](https://stockmarketstack.com/guides/cusip-isin-figi-identifiers).

## Why it matters here

Three questions to ask of any price source on this site, and they are the questions the cards try to
answer. Which adjustment convention does the adjusted series use? Can I get unadjusted prices
alongside it? And does the history keep securities that have stopped trading, or does it quietly
drop them?

Vendors built around this problem say so, and they are a different shape from an API that happens to
return a close price. [Norgate Data](https://stockmarketstack.com/tools/norgate-data) and [CSI Data](https://stockmarketstack.com/tools/csi-data) sell
adjusted-and-unadjusted history with the corporate actions as their own product; a general market
data API usually gives you one adjusted series and no way to see the factors behind it. If a number
has to reconcile against a statement, you need the unadjusted print; if it has to be comparable
across a decade, you need the adjusted one; if both, you need a source that keeps both.

[Why adjusted close differs between providers](https://stockmarketstack.com/guides/why-adjusted-close-differs) works through the
arithmetic and the specific conventions, and
[how to download historical prices in Python](https://stockmarketstack.com/how-to/download-historical-prices-in-python) is where
this bites first in practice.

## Where you will meet this

- [Norgate Data](https://stockmarketstack.com/tools/norgate-data.md)
- [CSI Data](https://stockmarketstack.com/tools/csi-data.md)
- [FirstRate Data](https://stockmarketstack.com/tools/firstrate-data.md)
- [Tiingo](https://stockmarketstack.com/tools/tiingo.md)
- [ClickHouse](https://stockmarketstack.com/tools/clickhouse.md)
- [Databento](https://stockmarketstack.com/tools/databento.md)
- [DolphinDB](https://stockmarketstack.com/tools/dolphindb.md)
- [Apache Druid](https://stockmarketstack.com/tools/druid.md)
- [DuckDB](https://stockmarketstack.com/tools/duckdb.md)
- [InfluxDB 3](https://stockmarketstack.com/tools/influxdb.md)
- [MX](https://stockmarketstack.com/tools/mx.md)
- [QSTrader](https://stockmarketstack.com/tools/qstrader.md)

## FAQ

### Why do two providers give different adjusted closes for the same day?

Because adjustment is a convention, not a measurement. Providers differ on whether dividends are reinvested, on the factor arithmetic and rounding, on which distributions count, and on how far back they restate. Each can be internally consistent and still disagree with the others by a few per cent over a decade.

### Do I need unadjusted prices as well?

If you are reconciling against a broker statement or a tape print, yes — those show what actually traded. Adjusted series are for comparing a price across an event. Any vendor that cannot give you both is making a decision on your behalf that you cannot undo later.

## Sources

1. [FINRA Rule 11140. Transactions in Securities "Ex-Dividend," "Ex-Rights" or "Ex-Warrants"](https://www.finra.org/rules-guidance/rulebooks/finra-rules/11140) — Financial Industry Regulatory Authority, 2024-05-28
2. [17 CFR 240.10b-17 Untimely announcements of record dates](https://www.ecfr.gov/current/title-17/chapter-II/part-240/subpart-A/subject-group-ECFRbda83517ce4377f/section-240.10b-17) — Electronic Code of Federal Regulations, read 2026-09-14
3. [Daily List Product Description](https://www.nasdaqtrader.com/Trader.aspx?id=dailylistpd) — Nasdaq, Inc., read 2026-09-14
4. [Corporate Actions ISO 20022 Messaging Specifications](https://www.dtcc.com/asset-services/corporate-actions-processing/iso-20022-messaging-specifications) — The Depository Trust & Clearing Corporation, read 2026-09-14

*Last updated 2026-09-21. A reference page, corrected in place — not a dated post.*