Three other common strategies you may hear traders refer to include momentum trading (buying shares of very fast growing companies and selling them for a profit before they inevitably peak in price), swing trading (using technical analysis to identify a trading range, and then buying and selling shares as the stock trades within that range), and penny stock trading (buying shares of very small companies whose stocks trade for less than $1 a share).
In margin buying, the trader borrows money (at interest) to buy a stock and hopes for it to rise. Most industrialized countries have regulations that require that if the borrowing is based on collateral from other stocks the trader owns outright, it can be a maximum of a certain percentage of those other stocks' value. In the United States, the margin requirements have been 50% for many years (that is, if you want to make a $1000 investment, you need to put up $500, and there is often a maintenance margin below the $500).
The stock market is one of the most important ways for companies to raise money, along with debt markets which are generally more imposing but do not trade publicly. This allows businesses to be publicly traded, and raise additional financial capital for expansion by selling shares of ownership of the company in a public market. The liquidity that an exchange affords the investors enables their holders to quickly and easily sell securities. This is an attractive feature of investing in stocks, compared to other less liquid investments such as property and other immoveable assets.
This event demonstrated that share prices can fall dramatically even though no generally agreed upon definite cause has been found: a thorough search failed to detect any 'reasonable' development that might have accounted for the crash. (Note that such events are predicted to occur strictly by chance, although very rarely.) It seems also to be the case more generally that many price movements (beyond that which are predicted to occur 'randomly') are not occasioned by new information; a study of the fifty largest one-day share price movements in the United States in the post-war period seems to confirm this.
^ Goetzmann, William N.; Rouwenhorst, K. Geert (2008). The History of Financial Innovation, in Carbon Finance, Environmental Market Solutions to Climate Change. (Yale School of Forestry and Environmental Studies, chapter 1, pp. 18–43). As Goetzmann & Rouwenhorst (2008) noted, "The 17th and 18th centuries in the Netherlands were a remarkable time for finance. Many of the financial products or instruments that we see today emerged during a relatively short period. In particular, merchants and bankers developed what we would today call securitization. Mutual funds and various other forms of structured finance that still exist today emerged in the 17th and 18th centuries in Holland."
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