A stock split converts each existing share into several, with the price adjusted by the same factor. Nothing about the company or the shareholder's stake changes, yet splits are consistently treated as news.
The arithmetic is a redenomination
In a split, every holder receives additional shares in proportion to what they held, and the market price adjusts downward by the same ratio.
A shareholder's percentage of the company, their share of earnings and the value of their position are identical before and after.
The company's market capitalisation is unchanged because the same total value is now divided into a larger number of smaller units.
Accessibility was the historical justification
When shares traded only in round lots of a hundred, a high price put a single lot beyond the reach of many individual investors.
Splitting brought the lot size back into a purchasable range, which genuinely widened the potential shareholder base.
Fractional share trading has largely removed that constraint, since a buyer can now acquire any dollar amount regardless of the price per share.
Splits carry a signal about management confidence
Boards typically split after a sustained price rise and are reluctant to do so if they expect the price to fall back afterwards.
The announcement therefore conveys information about how management views the durability of the run, even though the split itself adds nothing.
Any price reaction is a response to that signal and to increased attention, not to the mechanical change in share count.
Index membership can be affected in specific cases
Most indices weight constituents by market capitalisation, which a split leaves untouched, so membership and weight are unaffected.
Price-weighted indices are the exception, since a lower share price reduces a company's influence on the index without any change in its size.
That has occasionally made splits consequential for index composition in ways that have nothing to do with the underlying business.
Reverse splits run the mechanism backwards
A reverse split consolidates shares into fewer units at a proportionally higher price, again leaving total value unchanged.
It is generally used to lift a price above an exchange listing minimum or above thresholds that some institutions apply to what they will hold.
Because the circumstances prompting it are usually poor, a reverse split tends to be read as a negative signal even though the arithmetic is neutral.