For example, see Yahoo's stock: http://finance.yahoo.com/q?d=t&s=YHOOThe horizontal line appears toward the top.Yet Google's is somewhere in the middle: http://finance.yahoo.com/q?s=googThank you in advance! :)

The red horizontal line on Yahoo! Finance graphs represents the previous days closing price for the firm.

It is the previous closing price of the stock.Remember that many stocks trade in the after-market. Also, the market makers/specialist will decide the opening price range of a stock. This means that the opening price of the stock may not match the previous close.Keep in mind that lines across price graphs will not always be describing the previous close. Someone could very easily decide that it is important to show the previous week's high relative to the share price... or the opening print... etc.Happy trading.