汉乐府民歌《饮马长城窟行》写道:“客从远方来,遣我双鲤鱼。呼儿烹鲤鱼,中有尺素书。”什么是“双鲤鱼”呢?闻一多《乐府诗笺》云:“双鲤鱼,藏书之函也。其物以两木板为之,一底一盖,刻线三道,凿方孔一,线所以通绳,孔所以受封泥。……此或刻为鱼形,一孔以当鱼目,一底一盖,分之则为二鱼,故曰双鲤鱼也。”闻一多的说法,本于近人傅振伦的《简策说》中云:“函者,咸也。咸者,缄也。凡封缄者,始谓之函。盖凡封藏物者曰缄、曰椟、曰函。其物为木板,上刻线三道,凿方孔一,线所以通绳,孔所以封泥。《新 Han Yue Fu folk song “Yin Ma Great Wall line” wrote: “From afar, sent me double carp. Calling children cooked carp, there is a scale book.” What is “double carp”?笺 “cloud:” double carp, also a collection of letters also .Its two wooden boards to it, one end of a cover, engraved three, chisel a hole, so the line through the hole, so the hole is sealed mud .... This Or engraved as a fish, a hole to be a fish head, a bottom of a cover, the second is divided into two fish, it said double carp too. “Wen Yiduo’s argument, the recent Fu Zhenlun’s” Jane policy “in Cloud: ”The letter, salty also. The salty person, but also .Where the seal of the person, the beginning of that letter." Gai where the possession of those who say the seal, said 椟, said letter .Its material for the board, the engraved three , Chisel a hole, so the line through the rope, hole sealing mud