The hourglass theorem is the triple meaning of similar triangles butterfly principle AB=CD AB=CD to be learned in junior high school;

First, the time period (number of candles) of AB segment trend should be infinitely close to the time period (number of candles) of CD segment trend.

Second, the spread of AB segment should be infinitely close to CD segment.

Thirdly, the golden section callback ratio at point C of AB trend and the golden section callback ratio predicted at point D of BC trend are infinitely close to AB=CD (callback ratio/callback ratio) in the comparison table.