Basic Principles for Locating Materials by LC Call Number

Library of Congress call numbers follow three principles of organization: alphabetical order, numerical order, and decimal order. All numbers preceding the first decimal in an LC call number are read in counting order. Numbers that follow the first and any subsequent decimals are read according to decimal value. This one point in the arrangement can be confusing to a person unfamiliar with the LC classification. The following rules present general guidelines for interpreting LC numbers and shelf locations.  

1--Initial letters of a call number are followed alphabetically:  A, B, C, D, etc.

2--Two letter classifications follow single letters alphabetically in combinations:  P, PA, PB, PC, PD, PE, etc.

3--Numbers before the first decimal in a call number are read in counting order:  Q1, Q2,... Q10, Q11,..., Q100, Q101, etc.

4--If a call number has a decimal qualification attached to it, the attached number is read in decimal order:  QA75.1, QA75.13, QA75.2, QA75.275, etc. 

This part of the call number seems to be problematic for some library users. Looking at the decimal number, a person might think: "The number 13 is of greater value than the number 2, so the number 2 should come first in a call number arrangement. Thus, QA75.2 should precede QA75.13." This is not the case, however, because the numbers following the decimal are read as decimals. Think of the numbers in terms of money if this concept gives you any difficulty. A quarter ($.25) has less value than a half dollar ($.50). If you were counting money proceding from least to greatest, $.25 would precede $.50. If it helps you to read the numbers, add balancing zeroes to the single and double digit decimals to make them appear equivalent to their three digit counterparts. For example: QA75.100, QA75.130, QA75.200, QA75.275. 

5--In call letters with a single decimal, any letters following the decimal are read alphabetically:  QA75.A54, QA75.B23, QA75.C34, etc.

In call letters with two decimals, any letters following the second decimal are also read alphabetically:  QA75.13.A54, QA75.13.B23, QA75.13.C34, etc.

6--Probably the most confusing rule of call number order is that numbers following a letter that follows a decimal are also read in decimal order, in spite of the presence of the letter:  PS1.A2, PS1.A23, PS1.A3, PS1.A35, PS1.A4 and similarly: QA75.13.A1, QA75.13.A135, QA75.13.A2, QA75.13.A254, QA75.A3

If this pattern confuses you, try using the same tip as in number 4 above, add zeroes to the numbers to make them appear equal in value. 

An Extended Example of LC Call Number Arrangement

The following call numbers are in LC call number order, just as they should appear on the library's shelves. 

PS3511.A86A16 
PS3511.A86A6 
PS3511.A86A65 
PS3511.A86A673 
PS3511.A86A6745 
PS3511.A86A675 
PS3511.A86A6768 
PS3511.A86A7 
PS3511.A86A8 
PS3511.A86A86313 
PS3511.A86A864 
PS3511.A86A87 
PS3511.A86B439 
PS3511.A86B5 
PS3511.A86F3 
PS3511.A86F33 
PS3511.A86F37 
PS3511.A86G6 
PS3511.A86G634 
PS3511.A86H3 
PS3511.A86I5 
PS3511.A86I537 
PS3511.A86J4 
PS3511.A86K6 
PS3511.A86L5 
PS3511.A86L578 
PS3511.A86M28 
PS3511.A86M34 
PS3511.A86M6 
PS3511.A86N4 
PS3511.A86P9 
PS3511.A86R4 

Should you still have difficulty locating materials by call number, ask for assistance at the library's Reference Desk. 

Updated 11/08.

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