Q. How do LC Call Numbers correspond to subjects?

Thanks for asking this important question! The numbers you see on the spine of library books represent the US Library of Congress (LC, in library-speak) Classification System. Most academic/research libraries in the US use this system to determine the order of shelved items so that materials on the same topic can be browsed together by a library user.

An overview of the Library of Congress Classification System can be found online at loc.gov/catdir/cpso/lcco. You will note that each major heading (letter of the alphabet) is further divided into sub-classes for greater precision. For example, Class Q = Science and comprises numerous subclasses representing each scientific discipline: QA = Mathematics; QB = Astronomy; QC = Physics . . . QE = Geology; QR = Microbiology; and so on.

Finally, please note that materials in certain LC classes may be held in other Caltech Library locations. For example, some QC materials may be found in the Cahill Library; QE's are mostly held in the Geology Library (in N Mudd). The Library Catalog record for any item displays not only the Call No. (representing subject area) but also the holding location where the item is shelved.