It is said that Dharma has four legs (pillars) and it is depicted in the form of vrushabham ([nandi](https://en.wikipedia.org/wiki/Nandi_(bull))). What are those four legs? As far as I know, cow's four legs are also seen as four legs of Dharma.

Dharma has 4 legs in [Satya yuga](http://en.wikipedia.org/wiki/Satya_Yuga), 3 in [Treta](http://en.wikipedia.org/wiki/Treta_Yuga), 2 in [Dwapara](http://en.wikipedia.org/wiki/Dwapar_Yuga) and just 1 leg in [Kali yuga](http://en.wikipedia.org/wiki/Kali_Yuga). How to understand this?

Does it merely mean the percentage of righteousness in an individual, righteous people in a society or a theoretical combination of some practices?