Yer-su (yer-sub) is a paired word from the Orkhon inscriptions. It means the homeland itself and, at the same time, the spirits that guard it. Every mountain, every spring, every pass has an owner, and whoever passes asks its leave.
The asking survives: tying a cloth at a spring, leaving a stone at a pass, hanging a ribbon on a tree. The spirits are not evil, but they have no patience for disrespect.
Balbal's Elements suit is the four legs of this idea: Fire, Wind, Water, Earth. All four keep the same name in every realm — because fire is fire everywhere.