# What are the CIP rules for cyclic substituents?

Here's what I read on Wikipedia (section "Cycles"):

To handle a molecule containing one or more cycles, one must first expand it into a tree (called a hierarchical digraph by the authors) by traversing bonds in all possible paths starting at the stereocenter. When the traversal encounters an atom through which the current path has already passed, a ghost atom is generated in order to keep the tree finite. A single atom of the original molecule may appear in many places (some as ghosts, some not) in the tree.

I don't understand how to assign CIP priorities to cyclic systems, for example, consider this:

How do we decide the CIP priorities? What does that paragraph I found on Wikipedia really mean? Could someone please give a detailed explanation?

• @AvatarShiny The ghost atom is counted as an atom of the same atomic number as its non-ghost counterpart but connected to nothing. Taking the fourth atom of one of the green chains and comparing it to the fourth atom of one of the red ones, both have C, H, H. We need to dive down another layer to find a difference. The red C (fifth in the chain) is connected to C, H, H. The green C is a ghost connected to nothing. Since $\ce{C}>\text{nothing}$, the red chain wins. – Jan Jan 19 '18 at 8:52