Most print books in the library are organized by the Library of Congress call number, which groups similar subjects together on the shelves.
When you are looking for a book, start with the top line of the call number; all of the books are arranged alphabetically by this top line. For example:
B, BR, BS,...C, CA, CB
Then look to the number line next...
B B B B
3 21 221 3021
If these top two lines are identical, look to the third line where books with the same letter are arranged by decimal number...
PN PN PN
70 6321 6321
.P41 .E295 .F44
Finally, if the top three lines are identical look to the fourth line which contains a letter followed by numbers arranged by decimal number (Note: this works like the third line except that the numbers are treated as a decimal even though there is no visible decimal point)...