Grotrian diagram for the two-term system with hyperfine structure (HFS) considered to model the K I D lines. The D₁ and D₂ lines arise from the transitions between states of the same lower fine structure (FS) level 4s ²S_1/2 (ground term) and the states of the and levels, respectively. The atomic system has nuclear spin I = 3/2, and thus the D₁ line consists of four HFS components (red lines), whereas the D₂ line consists of six HFS components (blue lines).