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A stock split or stock divide increases the number of shares in a public company. The price is adjusted such that the before and after market capitalization of the company remains the same and dilution does not occur. Options and warrants are included.
Take, for example, a company with 100 shares of stock priced at $50 per share. The market capitalization is 100 × $50, or $5000. The company splits its stock 2-for-1. There are now 200 shares of stock and each shareholder holds twice as many shares. The price of each share is adjusted to $25. The market capitalization is 200 × $25 = $5000, the same as before the split.
Ratios of 2-for-1, 3-for-1, and 3-for-2 splits are the most common, but any ratio is possible. Splits of 4-for-3, 5-for-2, and 5-for-4 are used, though less frequently. Investors will sometimes receive cash payments in lieu of fractional shares.
It is often claimed that stock splits, in and of themselves, lead to higher stock prices; research, however, does not bear this out. What is true is that stock splits are usually initiated after a large run up in share price. Momentum investing would suggest that such a trend would continue regardless of the stock split. In any case, stock splits do increase the liquidity of a stock; there are more buyers and sellers for 10 shares at $10 than 1 share at $100. Some companies have the opposite strategy: by refusing to split the stock and keeping the price high, they reduce trading volume. Berkshire Hathaway is a notable example of this.
Other effects could be psychological. If many investors believe that a stock split will result in an increased share price and purchase the stock the share price will tend to increase. Others contend that the management of a company, by initiating a stock split, is implicitly signaling its confidence in the future prospects of the company.
In a market where there is a high minimum number of shares, or a penalty for trading in so-called odd lots (a non multiple of some arbitrary number of shares), a reduced share price may attract more attention from small investors. Small investors such as these, however, will have negligible impact on the overall price.
Effect on historical charts 
When a stock splits, many charts show it similarly to a dividend payout and therefore do not show a dramatic dip in price. Taking the same example as above, a company with 100 shares of stock priced at $50 per share. The company splits its stock 2-for-1. There are now 200 shares of stock and each shareholder holds twice as many shares.
The price of each share is adjusted to $25. Based on this example you might expect to see the stock dropping from $50 to $25.
So what is done is something called adjusted close price. This adjusted close price will take all the closing prices before the split and divide them by the split ratio. So when you look at the charts it will seem as if the price was always $25. Both the Yahoo! historical price charts and the Google historical price charts show the adjusted close prices.
See also 
- Reverse stock split
- Share dividend
- Share repurchase also known as stock buyback
- Berkshire Hathaway, which has never had a stock split, has at times been valued at over US$140,000 per share.
- Market depth
- Online database predicting over 2,000 stock splits
- Stock split calendar showing prices pre-split and post-split
- Profiting From Special Situations Like Stock Splits
- Stock split calendar for U.S. companies
- Stock split and reverse split examples for shareholders
- Swiss stock split calendar for European companies and worldwide companies incl. U.S. companies (in German)
- Useful resource for historical split dates and prices of all stocks