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Materials in a library are assigned and organized by their Call Numbers, which are determined by their subject and disciplinary focus. Sciences, for example, is assigned the letter Q. Fiction is assigned POP and the last name initial of the author.

These Call Numbers are unique, and they are be placed on the spine of a book.  The Call Number of a book can be determined by searching the library’s online catalogue.  Call Numbers are the means by which books may be located on the library shelves.  The Petroleum Institute Library, as many other academic libraries, uses the Library of Congress Classification, which are a series of letters and numbers.  For example you are looking for this book:

Nanotechnology for Environmental Decontamination

At first, these call numbers appear very confusing.  There is, however, a simple logic to how they work.  This can best be explained by breaking the call number up into parts.

The first part is the leading letters, in this case: TD TD192 .N36 2011 V.2 These leading letters are the primary means of organization, and they are in alphabetical order.  That is, all call numbers starting with A will come before all call numbers beginning with B, and those will come before C, those before D, and so on through to Z.  That means that T call numbers will come before TA call numbers, and those will come before TD call numbers, and again through to TZ call numbers, which will come before any Z call numbers. 

The second part of the call number is the first series of numbers, in this case: 192 TD192 .N36 2011 V.2 These numbers work just like regular numbers, and are organized in numerical order within the confines of the above mentioned alphabetical order.  That is, TD 1 will come before TD 2, which will come before TD 192. 

The third part of the call number is the secondary letter (or letters), in this case:  N TD192 .N36 2011 V.2 This, again, is in alphabetical order so TD 192 A would come before TD 192 AR and that would come before TD 192 B.

The fourth part of a call number is the second set of numbers, in this case: 36 TD192 .N36 2011 V.2 This number behaves like a decimal number.  Therefore any one of these number starting with a 3  will come before any of these numbers starting with a 4.  For example, TD192 .N36 will come before TD192 .N3654 , which will come before TD192 .N36996 . 

The next part of the call number is the year, in this case: 2011 TD192 .N36 2011 V.2 
Years are listed in numerical order so TD192 .N36 2011 will come before TD192 .N36 2015. 
Years will rarely come into play, unless you are looking for a specific or newer edition of a resource.

The last part of a call number is the volume number, in this case: V.2 TD192 .N36 2011 V.2. This number denotes the volume number of multi-volume works.  Not all books will have a volume number. 
Volumes are organized in ascending order.  V.1 before V.2, etc. 

Some books will also have a copy number, denoted by C.#.  This indicates that the library has more than one copy of the same book e.g C.2 is a second copy. 

For Further information check this LibGuide