Let's find books about butterflies. To find the classification number for butterflies, we need to start with the class for natural sciences: the 500's. This means that the first number of the call number will be a 5. Let's look at the ten divisions of this 500 class:
Butterflies will be classified under the Zoological Sciences - 590 - so we know that the second number of the call number will be a 9 and the first two numbers are 59. Let's look at the ten divisions of the 590's to find the next number.
Insects, including butterflies, would be under 595, so the first three numbers of the call number are 595.
The 595's are further divided by the use of decimals to specify what type of insects:
So the Dewey Decimal Classification Number for butterflies is 595.789. Beginning to get the picture? The more numbers, the more specific.