One of the basic tenets put forth by Charles Dow in the Dow Theory is that security prices do trend. Trends are often measured and identified by "trend lines." A trend line is a sloping line that is drawn between two or more prominent points on a chart. Rising trends are defined by a trend line that is drawn between two or more troughs (low points) to identify price support. Falling trend-s are defined by trend lines that are drawn between two or more peaks (high points) to identify price resistance.

Interpretation :

A trend line is a sloping line drawn between two prominent points on a chart.  Rising trend lines are usually drawn between two troughs (low points) to illustrate price support while falling trend lines are usually drawn between two peaks (high points) to illustrate upside price resistance.

The consensus is that once a trend has been formed (two or more peaks/troughs have touched the trend line and reversed direction) it will remain intact until broken.

That sounds much more simplistic than it is!  The goal is to analyze the current trend using trend lines and then either invest with the current trend until the trend line is broken, or wait for the trend line to be broken and then invest with the new (opposite) trend.

The following chart  displays two trend lines.  The first trend line is a "falling" trend line.  Note how it is drawn between successive peaks in the price.  The second trend line is rising.  It is drawn between successive troughs.

One benefit of trend lines is they can help distinguish intuitive decisions ("I think it's time to sell...") from analytical decisions ("I will hold until the current rising trend line is broken").  Another benefit of trend lines is that they almost always keep you on the "right" side of the market.  When using trend lines, it's difficult to hold a security for very long when prices fall just as it's hard to be short when prices rise--either way the trend line will be broken.