The expansion of trade during the song dynasty eventually led to

The expansion led to the invention of a money system.
The expansion allowed many traders from China to exchange goods and services with traders from other nations.
In order to make trading process became fairer to both sides, a money system is needed as a form of valuation on a certain products/services.

They made a checks and balance system to equal power within the branches. the "check" meant that one branch would watch one branch and "balance" them if they were too powerful. if you want more info on this, i would just google checks and balance system.