A symbol of "Double Happiness"(囍) is something that cannot be missed during a Chinese wedding- whether it may be a traditional one or among those which have a more contemporary set-up.
   It is usually printed on the invitation with the use of golden hot stamps. The symbol of Double Happiness is a declaration that the bride-to-be and the groom-to-be will be united. The symbol, which is composed of two standard Chinese characters used to signify "happiness", means that the couple and their families will now be "together". 
   Each of the characters that denote happiness is written as "xi" in Mandarin. In the case of the Double Happiness sign, the two "Xi" characters signify the happiness of the newlywed couple that are about to spend their lives together. Pronounced as "shuang-xi", the sign generally stands for marital happiness. Note that the Double Happiness sign is not used in regular Mandarin writing, but is only observed for marital union invitations and declarations.
The Origin of "Double" Happiness 
   The story of the Double Happiness sign originated from a student's journey during the Tang Dynasty. According to the story, the boy who was about to take a final examination fell sick on his way to the capital city. Luckily, an herbalist doctor and his daughter helped him. However, the girl did not just make him feel better, she also made him fall in love with her.
   Because the girl was in love with the boy, she wanted to make sure if the boy was her perfect match. And so, before the boy left for the capital city, the girl wrote a part of a rhyming couplet on paper, with the hopes that the boy can find its perfect match. 
   At the examination proper, the boy was able to bag the first place. When the emperor came to assess the boy's skill, he asked him to finish a couplet. Fortunately, the part of the couplet that the emperor gave the boy was the missing match to his love's rhyme. 
   The boy recited the part of the couplet that the girl wrote for him. Pleased with the young man's answer, the emperor made the boy one of his Ministers. Before taking his post, the boy went back to the girl and recited her couplet's match.
   They then got happily married. During their traditional Chinese wedding, the couple wrote the character "XI" twice on a red piece of paper. They posted it on the wall and since then, that double "XI" became the Double Happiness sign, symbolizing the "doubled" happiness that the couple felt because of their union.