What is Najia?

Najia is the system of mapping Heavenly Stems (天干) and Earthly Branches (地支) onto each line of a hexagram — literally "attaching stems and branches" (納甲). Each line receives a stem-branch pair (e.g., 甲子, 乙丑) which determines its Five Element attribute. The najia assignments follow fixed rules based on the hexagram's trigrams and are the foundation of all Liu Yao calculations.

Source: Jingshi Yizhuan (京氏易傳), Jing Fang