Two quotes from:
Technical Analysis of Stock Trends
Robert D. Edwards and John Magee,
8th edition, CRC Press, 2001
A Double Top is formed when a stock advances to a certain level with, usually, high volume at and approaching the Top figure, then retreats with diminishing activity, then comes up again to the same (or practically the same) top price as before with some pickup in turnover, but not as much as the first peak, and the finally turns down a second time for a Major or Consequential Intermediate Decline.
(page 134)

Standard & Poors 500 index (SPX weekly), 1970-2008, via bigcharts.com
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If prices on their recession from the second peak, drop through the Bottom level of the valley, a Reversal of Trend from up to down is signaled. And it is usually a sign of major importance. Fully confirmed Double Tops seldom appear at turns in the Intermediate Trend; they are characteristically a Primary Reversal phenomenon. Hence, when you are sure you have one do not scorn it. Even though prices may have already receded 20%, the chances are they have very much further to go befor they reach bottom.
(page 140)