The guideline of alternation states that if wave two of an impulse is a sharp retracement, expect wave four to be a sideways correction, and vice versa. Figure 1 shows the most characteristic breakdowns of impulse waves, both up and down. Sharp corrections never include a new price extreme, i.e., one that lies beyond the orthodox end of the preceding impulse wave. They are almost always zigzags; occasionally they are double threes that begin with a zigzag. Sideways corrections include flats, triangles, and double and triple corrections. They usually include a new price extreme, i.e., one that lies beyond the orthodox end of the preceding impulse wave.

Key Takeaway: Lack of alternation can tip you off to an acceleration.

For a complete description, please read Elliott Wave Principle: Key to Market Behavior by Robert Prechter and A.J. Frost.