A call number is simply an address so books (and other materials) can be located. Perkins Library uses Library of Congress classification to arrange books by subject. The first line of the call number is one, two, or three letters and is read alphabetically. The second line is made of numbers and is read numerically. The third line can be trickier. The letter is shelved alphabetically but the number following the letter is treated as a decimal number (e.g., .E4571 comes before .E461). If the top three lines are identical look to the fourth line. If it contains a letter followed by numbers, items are organized alphabetically by letter and then by decimal number.