Putting up door gods, or changing door gods, is an important custom among the Chinese during the Spring Festival. Door gods are pictures of deities posted on the door, where they can protect the household from evil.
The custom originated during the Tang Dynasty (618- 907 AD) when the Emperor Tang Taizong (599 - 649) honored two loyal generals by having their portrait painted on his front door, due to their bravery in fighting intruders and evil spirits.
Ordinary families soon adopted the imperial custom, putting woodblock prints of the ever-vigilant generals on their front gates in the hope of attracting good luck and fending off evil spirits.
The door god business soon spread throughout China, and Chinese New Year is the time for posting door god portraits.
The door gods usually face each other in pairs; it is considered bad luck to place the figures back-to-back.