The CLI is defined by the standard ECMA-335. This is a  .NET infrastructure that allows applications written in multiple programming languages to operate many different environments without the need to modify the program code. The CLI consists of: a Portable Executable (PE) file format, a CTS (Common Type System), an extensible metadata system, an CIL (Common Intermediate Language), a FCL (Factored base class library),  and access to the underlying operating system (Win32).