It is also most reliable when found in an uptrend as well. Eventually, the market begins to slow down and the forces of supply and demand are generally considered in balance. Tentative buying re-emerges and the market rallies once more, but fails to take out the previous high. Buying dries up and the market tests the downside yet again. Your trendline for this pattern should be drawn from the beginning neckline to the continuing neckline.
Volume generally follows the price higher on the left shoulder.
However, the head is formed on diminished volume indicating the buyers aren't as aggressive as they once were. New selling comes in and previous buyers get out. The pattern is complete when the market breaks the neckline. The head and shoulders pattern can sometimes be inverted. The inverted head and shoulders is typically seen in downtrends.