Compound coaxial stacks. An illustration for the three general situations of compound coaxial stacks, where 5' and 3' indicate the backbones from the 5' end and to the 3' end of the sequence, respectively. In the left structure, the stacks (X, Y) and (Z, X) are nested stacks. In the middle structure, the stack (X, Y) is nested while (Z, X) is parallel. In the right structure, the stack (X, Y) is parallel while (X, Z) is nested.