Ruler, ruler, ruler book. .
Also, I remember a poem that left me a pair of carp with a ruler book in it.
Double carp is a woodcut carp made of two wooden boards, which is equivalent to an envelope. What about the size element? This is a letter written in ordinary silk ~
The documents of Han Dynasty are mainly bamboo slips, which are made of bamboo and wood ~
There was a paper letter at the end of the Han Dynasty. =~