Literati like to study literary objects and carve their favorite words or personal poems on them to entertain themselves. Someone once collected a silver reed inscribed by the great calligrapher Zhao. Yin Gui sawed a rectangular nanmu at a right angle, like a kitchen knife, and carved the words "hook the string for the edge". That is to say, there is a right triangle between the two sides of a right angle, the short side is a hook and the long side is a strand. In ancient times, when measuring the height of the sun with a vertical pole, the sun shadow was a hook and the bamboo pole was a strand. Zhao skillfully used this point in "Leading Ghosts", and the literary interest jumped to the page. Yin Gui, also known as printing moment, gauge is a tool for drawing circles, and moment is a ruler for drawing right-angled squares and rectangles, which is more appropriate. But it's usually called a ghost. Maybe it's because some ghosts are made of circles and cut out a right angle. In Tianheng Printing, there is a picture of a stone corner with a small seal at the end, which is quite elegant. There are also two square copper circles. Divide the flat round copper into four parts, cut off a quarter, leave a right angle and carve it on the copper surface. It is timely to call it a ghost.