Call numbers are usually near the bottom of the book spine facing out from the shelf. Thus, it can be easily found when walking down the aisle of a library. Most libraries also have an online catalogue that can help you easily find the call number for a book based on the title, author or subject matter.
Call numbers will usually have multiple lines depending on how many books are in the library and the extent to which they need to classify a specific book. Always read a call number from the top down, line by line. So for example, in the Library of Congress Classification for call numbers, the most widely used call number system, the first line consists of letters while the second is a whole number. The third line is a combination of letters and numbers that follow a decimal point. The last line in this system is the year the book was published.
Each line in a call number classifies a book into a certain section. In the Library of Congress Classification system, the letters and number in the first and second lines represent the subject of a book. Next, the combination of letters and numbers in the third line of the call number stand for the author's last name. And, as previously stated, the last line stands for the date of publication. This is useful when an author has a common last name and there are many publications over the span of many years.
Call numbers are organized in alphabetical and sequential order. So in the Library of Congress Classification, the books would first be organized alphabetically based on the letters of the first line. Next, each combination of the first line would be further organized sequentially based on the number in the second line. Thus, you can narrow down where your book is located by first going to the section with the same letters in the first line of your book's call number. Then, in that section, you would count until you get to the number on the second line. This method is used line by line for all the lines available in that call number system.