Wave D
Wave D is the fourth leg of a triangle, the five-wave sideways pattern labeled A-B-C-D-E that appears in Wave 4 or Wave B position. Like every triangle leg except sometimes the first, Wave D subdivides into three waves and stays inside the converging trendlines. In a contracting triangle it is shorter than Wave C, which is what keeps the pattern narrowing toward its apex. Wave D matters because it tells you the triangle is maturing: once D completes, only Wave E remains before the market breaks out in the direction of the larger trend. Traders who recognize Wave D early can prepare the breakout trade instead of being surprised by it. A common mistake is treating the push that forms Wave D as the start of the breakout itself. The structure gives it away: Wave D is corrective and overlapping, while a real breakout is impulsive.
EURUSD builds a Wave 4 triangle on the 4-hour chart. Wave C bottoms at 1.0820, then price grinds up to 1.0885 in a clearly three-wave advance that stalls below the upper trendline. That push is Wave D. You now watch for one final dip (Wave E) before the Wave 5 breakout.