The core skeleton is an anthraquinone, consisting of three six-membered aromatic rings fused linearly in a row. The central ring contains two unique carbons (not shared with the adjacent rings), and each of these carbons double bonds to an external oxygen.

On the rightmost benzene ring of the anthraquinone:
- Number the ring junction carbons as C1 and C2, then continue clockwise around the ring as C1-C2-C3-C4-C5-C6.
- C3 and C4 are the fusion points with another benzene ring.
- C5 is bonded directly to an additional benzene ring, and this extra benzene ring carries a methyl group at the position para to the anthraquinone linkage.