From The Free Forex Encyclopedia
Descending trendlines are a variety of trendlines, one of the most fundamental tools for technical analysis. Descending trendlines are simply trendlines with a negative slope, indicating falling prices. There are two types of descending trendlines: descending top trendlines, in which the high prices for an asset are falling, and descending bottom trendlines, in which the low prices for the asset are falling.
The rules for trading using descending trendlines are the same as the rules for trading with trendlines in general. A descending top trendline is a measure of the resistance to an asset's price, and traders consider a break in price through the descending top to be buy signal for the asset. A descending bottom trendline is a measure of the support in an asset's price, and traders consider a break in price through the descending bottom to be a sell signal for the asset. Many traders consider it necessary for additional signals to appear on the chart before a broken descending trendline is confirmed, and before those traders will take the appropriate market action.