骑

The jì reading is classical / literary and survives mainly in Tang-Song poetry and historical prose, where 骑 names a horse-with-rider as a counted unit (one jì = one mounted soldier). Modern Mandarin has largely collapsed both readings into qí - Du Mu's 一骑红尘 is the classroom example where teachers still insist on jì, but most mainland speakers now read it qí. Taiwan dictionaries preserve jì more strictly.

The verb / noun split is ancient: the same graph was read with different tones depending on whether you were doing the riding (verb, level tone) or being counted as a rider (noun, departing tone). The 一骑红尘妃子笑 line refers to a courier galloping fresh lychees from Lingnan to Chang'an for Yang Guifei - one horse-and-rider counted as a single jì.