If you are wondering about the strange title of this post, it's how librarians think of the alphabet. New York gets put before Newark, because a space takes precedence over an a.
Some libraries still put books beginning with Mc and Mac before books beginning with, say, Mab. BHPL just switched to shelving Mcs in "regular person" order a year or two ago: in between the Ma's and the Me's. And I can't remember anymore whether St. is supposed to be shelved under Saint or St.
These are the kinds of things that drive fiction shelvers crazy. In the nonfiction section, the Dewey decimal call numbers ought to be easy, right?
Not if you got used to Library of Congress classification while you were in college. LC treats the numbers that come after letters in call numbers as whole numbers (so PS3551 comes before PS3560). Dewey treats numbers after the decimal point like decimals, so something ending in 412.19 comes between 412.1 and 412.2.
If you aren't confused yet, consider the fact that Harvard and Princeton have their own special classification systems, Widener and Richardson, respectively. But most of their books are classified by the Library of Congress system.