Call numbers usually have four or five parts. The way to read a call number is to start at the top. The first line of the call number contains the Class Letters. In this case, the class letters are TR, which is for books about photography. The third line is the Cutter Number.
Most books have only one cutter number, but some have two.
Cutter numbers are used to put books in alphabetical order within class numbers. The numbers are assigned to ensure proper alphabetical order. The final line of the call number is the date of publication of the book. Some older books do not have the year in the call number, but almost all newer materials do. Five line call numbers have two cutter number lines.