Double Bottom: how does this pattern form, what market psychology does it reflect, and how reliable is it in practice? Give a real example only where one is genuinely well known, and say so plainly when it is not.

The double bottom is the "W": two lows at approximately the same price with a rally between them, and a neckline drawn across that intervening high. It marks a price at which demand reliably appears — the first low is where buying interest first overwhelms the decline, the rally fails, and price returns to test whether that demand was a one-off; when the second test holds at roughly the same level, it confirms the bid is real. The S&P 500's late-2008 and March-2009 lows are sometimes described as a double bottom before the subsequent recovery, though the two lows were roughly five months apart and of different depths, which stretches the classic symmetric definition somewhat. The second low holding on visibly lighter volume than the first is the constructive version. Confirmation is a close above the intervening high; invalidation is a new low beneath the pattern. The caveat: a double bottom can hold perfectly and still resolve into a long, flat range rather than an advance, because a level that stops a decline is not the same as a level that starts a trend. Confirmed structure tells you the downtrend paused; it does not promise the move it measures.