Stocks tend to move up or down in a series of zig-zag steps. As a result, a straightforward way of spotting a trend is to draw a line connecting the high points of a stock price over a certain period of time and extending it forward into the next weeks or months. A similar line will connect the low points and also extend that into the future. When a stock price reaches the extended top line, or resistance level, traders will conclude that it is time to sell all or some of their stock as it is ready to turn down. In turn, when the stock price descends to the lower line, or support level, traders will conclude it is time to buy.