What is the tick mark copy in calligraphy?

A practice method and means of brush calligraphy. Is to write only the skeleton of the middle line of the font with a pen tip. Such as fonts written by others. You write the center line on it repeatedly. Write one by one according to the strokes of each word and the center of the font axis. It is easy to master the font structure by practicing the pen tip like this. Practice for a long time, you will know, and you will know indifferently, and master the font structure imperceptibly. Another method is to tick the line and write the shape of the font, leaving a hollow in the middle, also called hollow characters. A professional practice is called double-slot font outline. Outline his font on the original font. Or it is a way to sketch and copy calligraphy by covering the original font with transparent paper.