Co Ngu Hanh is the name of Vietnamese festival flags that all have a red —or sometimes dark blue— indented border, with a thin white outer rim, a central square field and concentric squares in various colours.

These colours correspond to those of the Five Elements according to Chinese philosophy, are used interchangeably, and may appear is any order. Originally, these colours are red, green, yellow, white and black, though the black colour, which represents the element of water, is in Vietnam replaced by dark blue.

There are often but three concentric squares and one central square field in each flag. Occasionally, the central field of the flag may have one or more Chinese characters on it, usually in yellow onto a central field in red.