CLERESTORY

Clerestory is  term now restricted to the windows in an aisled nave, or to the range of wall in which the high windows are set but was formerly applied to any window or traceried opening in a church, e.g. in an aisle, tower, cloister, or screen, but . Sometimes these windows are very small, being mere quatrefoils or spherical triangles. However, is larger churches they are important features both of beauty and utility. The clerestory is especially used in churches where the division into nave and side aisles permits the introduction of light into the body of the church from above the aisle roofs.

CARLISLE CATHEDRAL
Picture taken from: www.varsitythreadsusa.com 

Acknowledgement: http://www.newadvent.org/cathen/04049a.htm