Simplest pattern from whole harmonic pattern is AB=CD
It is display of two equal legs (impulsive waves) and a correction.
For both the bullish and bearish versions of the ABCD chart pattern, the lines AB and CD are known as the legs while BC is called the correction or retracement.
If you use the Fibonacci retracement tool on leg AB, the retracement BC should reach until the 0.618 level. Then, the line CD should be the 1.272 Fibonacci extension of BC.
Fibonacci retracement levels for AB=CD are 0.50, 0.618 and 0.786. Strong trend movement will see only a shallow 0.382 retracement.
The perfect AB=CD pattern demand the of leg AB should be equal to the length of leg CD. The time that it takes for AB leg should be equal to the time it takes to CD