There are a number of data types available in the .Net framework for storing numbers with fractional parts. They are each appropriate for different situations, and using the wrong one can lead to errors in calculations.

Thinking About Accuracy

In some applications, you require that calculations involving numbers with fractional parts are exact. Examples of this are in financial applications, where losing the odd cent here and there in a calculation is unacceptable. Customers expect their accounts to be completely accurate, not to mention the tax man.

In other applications, you don't care about exact results, but you are interested in having an answer that is correct to a certain number of significant digits. This is the case in experimental physics, for example. When you make a measurement of something, you can only measure it as accurately as your equipment will allow you. Therefore, you have an inherent error in that value already, which is going to cascade through any calculations you do with it. This means that while your computed answer may be 5.1826, you may know that the values this answer was calculated from were only accurate to three significant digits, and so the last two digits in this result (the 0.0026) don't matter. If there is a computational error in them, so be it - it's not going to hurt us...