Internal loops are named according to the number of unpaired bases on either side of the stack. An x-y loop has x unpaired bases on the lower-numbered (5') side of the stack, and y unpaired bases on the higher-numbered (3') side of the stack. If x and y are equal, the internal loop is called a symmetric internal loop.
Main Article: Boosting
Special Internal Loops
The 1-1, 1-2, 2-1, and 2-2 loops each have special energy parameters.