A reader can trace a foot-noted lead with lightning-fast speed and determine whether the citation they’re following is something that requires their attention or not within a minute of seeing a footnote. Previously, this crucial step in the research process (following the breadcrumbs) was the most labor intensive, but with enhanced catalogs, digital surrogates, linked databases, and, most importantly, the hardware to bring all of these things to the fingertips of a library user, a library user becomes a walking catalog.

–Daryl Green, rare books librarian at the University of St Andrews, quoted in Future U: Library 3.0 has more resources, greater challenges | Ars Technica