A structural classification of loops extracted from known protein structures. The structural classification is based on the geometry and conformation of the loop. The geometry is defined by four internal variables and the type of regular flanking secondary structures, resulting in 10 different loop types. The new version of ArchDB features a novel, fast and user-friendly web-based interface, and a novel graph-based, computationally efficient, clustering algorithm. The current version of ArchDB classifies 149,134 loops in 5739 classes and 9608 subclasses.