HOOH energy level diagrams. The energy is given in K on the vertical axis, and at the bottom the quantum numbers K_a and τ are shown. To the left of each level the rotational quantum numbers J_Ka,Kc are listed. The upper diagram shows the levels with a nuclear spin weight of 1 (A_1 ↔ 3 and A_2 ↔ 4), while those levels with a spin weight of 3 (B_1 ↔ 3 and B_2 ↔ 4) are shown in the lower diagram. The allowed c-type transitions within each of the four ladders are indicated as arrows. The red arrows represent detected lines, while the red-dashed arrows indicate non-detections.