Who they were
A decisive general in the Tang founding under his father Li Yuan, Li Shimin took power in the Xuanwu Gate incident of 626, killing two of his brothers; his father then abdicated to him.
What they did
He tolerated and rewarded blunt remonstrance, most famously from the minister Wei Zheng, maintained a strong legal code, and expanded the examination system. After his defeat of the Eastern Turks in 630, steppe leaders honored him as “Heavenly Khagan”.
Legacy
The record of his court discussions, the Zhenguan Zhengyao, was studied by rulers across East Asia for centuries as a handbook of how to govern well.