Black border format (of traditional thread-bound books)

Dictionary definition: a format of thread-bound book mouth. The lines carved at the upper and lower ends of the center of the dish are called big black mouth and narrow black mouth. Mao Chunxiang's version of ancient books is often discussed. Why study the version? "As far as a book is concerned, there are many names, such as font, paper color, ink color, white mouth, black mouth, bilateral, unilateral, line payment, card note, preface and postscript, title page, title, etc."