Think win/win. Psychology is a huge aspect of trading. If you have a big winner on your hands and aren’t sure whether you should hold the shares to try for higher prices or sell them to lock in a profit, consider selling half and holding the rest with a stop loss (at worst) back at your original buy price. That way, if the stock drops back to your buy price, you still win because you sold half and made a profit. Similarly, if the stock shoot higher in price, you also win because you still hold half your original position. Heads you win, tails you win too. 🙂
The movements of the prices in a market or section of a market are captured in price indices called stock market indices, of which there are many, e.g., the S&P, the FTSE and the Euronext indices. Such indices are usually market capitalization weighted, with the weights reflecting the contribution of the stock to the index. The constituents of the index are reviewed frequently to include/exclude stocks in order to reflect the changing business environment.
Financial innovation has brought many new financial instruments whose pay-offs or values depend on the prices of stocks. Some examples are exchange-traded funds (ETFs), stock index and stock options, equity swaps, single-stock futures, and stock index futures. These last two may be traded on futures exchanges (which are distinct from stock exchanges—their history traces back to commodity futures exchanges), or traded over-the-counter. As all of these products are only derived from stocks, they are sometimes considered to be traded in a (hypothetical) derivatives market, rather than the (hypothetical) stock market.