CTS defines all of the basic types that can be used in the .NET Framework and the operations performed on those type. All this time we have been talking about language interoperability, and .NET Class Framework. None of this is possible without all the language sharing the same data types. What this means is that an int should mean the same in VB, VC++, C# and all other .NET compliant languages. This is achieved through introduction of Common Type System (CTS).