A trend line is drawn by connecting two or more swing points on a chart - typically rising swing lows (for an uptrend line) or falling swing highs (for a downtrend line). It’s a simple, visual way to represent the general direction of a price move, and many traders watch how price behaves as it approaches a trend line again, similar to a horizontal support or resistance level, except angled rather than flat.
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See how to draw one step-by-step in Trend Lines on TradingView: How to Draw and Use Them.