T-duality

http://iopscience.iop.org/1742-5468/2006/12/P12016/fulltext#SECTIONREF

 

T-Duality
This refers to the situation where one string theory compactified on a circle of radius R, and another string theory compactified on circle of radius 1/R describe the same physics. Therefore when one of the theories is on a very small circle the other theory is on a very large circle.

http://www.sukidog.com/jpierre/strings/glossary.htm