What does threading mean in calligraphy?

In a single character or upper and lower structure, suppose a vertical line passes through the center of gravity of the character, and the stroke strength on both sides of the line is evenly distributed. To practice the basic skills of calligraphy, we must first understand the three elements of calligraphy: brushwork, structure and composition. China's calligraphy is mainly represented by lines. In order to enrich line stippling, we should first pay attention to writing and master the methods of writing, such as weight, speed and integrity. This is the so-called "brushwork". Threading means that in a single character or an up-and-down structure, it is assumed that a vertical line passes through the center of gravity of the character, and the strokes on both sides of the line are evenly distributed.