What is the sky, foot, front, and order of books?

Tiantou refers to the paragraph between the headline of the top line of the body of a book and the edge of the paper above the block.
The position of the footer is opposite to the head of the sky. It refers to the foot of the bottom line of the body of the book and the edge of the paper between the bottom edge of the book.
The front mouth is also called a mouth or mouth side. Refers to the location of the reading edge of the burr relative to the creased edge.
The book of mouth refers to the position of the book section.