To understand Diagonals, you must first know the basic rules.
1. A diagonal is a 5 wave pattern
2. Each wave is made of Zig-Zag patterns
3. Unlike impulse waves, wave 4 is allowed to go into the territory of wave 1
Building from my previous post of the monthly time frame...
We know that Wave 1 & 2 of Monthly structure have concluded (Blue). Price is now building wave 3 (Blue) of the overall diagonal. This means that wave 3 (Blue) consist of zigzags as well.
The First push to the upside A (Green) has already concluded. Now, price is performing the retracement of A (Green), which must, in order to fulfill the rule of "Alternation", form a "Flat" of some sort upon conclusion.