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The books in Geisel Library are arranged by a system called Library of Congress Classification. The letters and numbers ("call numbers") on the label applied to the spine of each book represent this classificiation. The call numbers are used to keep the books in order, arranged by subject and sub-arranged by author. Books on religion are generally shelved with in the range of call numbers beginning with BL through BX.
BL = Religions (general and by region)
BM = Judaism
BP = Islam
BQ = Buddhism
BR = Christianity (especially history of)
BS = The Bible (text and works about)
BT & BV = Theology
BX = Christian Denominations (BX800+ = Catholicism; BX4795+ = Protestantism)
For more detailed information about how religion books are arranged within the BL-BX classification, go to the official Library of Congress page.