Meanwhile, for between the base pair such that but if contains no pair s.t. , the base is defined as accessible from .
For a secondary structure of a RNA molecule, if and both and are accessible from , the pair is said to be accessible from .
Assume that pairs and unpaired bases are accessible from . Then these bases within are said to constitute a -cycle or -loop closed by . The accessible pairs are called the interior pairs, whereas the closing pair is referred to as the exterior pair.