北地郡 / Beidi Commandery
北地郡 / Beidi Commandery enters the wiki through 《资治通鉴·周纪》85｜宣太后为何私通30年后暗杀了老情人？ as the administrative result of 秦国’s destruction of 义渠. After 芈八子 / 宣太后 kills the Yiqu king at 甘泉宫, Qin moves against the leaderless polity and establishes Beidi Commandery in 271 BCE.
The episode makes Beidi important as more than a conquered district. Together with 陇西郡 and 上郡, it gives Qin a connected northern frontier line. That line lets Qin turn the older problem of Yiqu pressure into frontier buffer management by commandery administration and defensive wall-building.
The host says the commandery seat was Yiqu county and links the likely old Yiqu city area to modern Ning County in Qingyang, Gansu. The exact archaeological-geography claim remains source-scoped to this episode.