Happiness comes in pairs: In the book Popular Chinese Customs Professor Lou Tzu-kuang notes that when people got married in ancient times, betrothal gifts would include a document recording all the details of the accompanying gifts.
喜事喜雙在「中國民俗」一書中，婁子匡教授提及，古代結婚時，完聘的聘禮中包括一張禮帖，記載著禮物的明細。