My brother-in-law's parents. What should I call them?

You and your brother-in-law are in-laws.

What should you call your brother-in-law's parents? China is a vast country with different names. According to the name of the old man in Beijing and Tianjin, your brother-in-law's father is called pro (pronounced green, pronounced green) or pro-uncle (pronounced white, pronounced white); Your brother-in-law's mother is your own mother. That's basically what it's called.