How Trends Work
Traders can identify a trend using various forms of technical analysis, including trendlines, price action, and technical indicators. For example, trendlines might show the direction of a trend while the relative strength index (RSI) is designed to show the strength of a trend at any given point in time.
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While the trend is up, traders may assume it will continue until there is evidence that points to the contrary. Such evidence could include lower swing lows or highs, the price breaking below a trendline, or technical indicators turning bearish. While the trend is up, traders focus on buying, attempting to profit from a continued price rise.
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When the trend turns down, traders focus more on selling or shorting, attempting to minimize losses or profit from the price decline. Most (not all) downtrends do reverse at some point, so as the price continues to decline, more traders begin to see the price as a bargain and step in to buy. This could lead to the emergence of an uptrend again.
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Trendlines should not be relied on exclusively to determine the trend. Most professionals also tend to look at price action and other technical indicators to help determine if a trend is ending or not. In the example above, a drop below the trendline isn't necessarily a sell signal, but if the price also drops below a prior swing low and/or technical indicators are turning bearish, then it might be.
Example of a Trend and Trendline