Side-by-Side Execution of .NET Framework 1.0 and 1.1

Side-by-side execution is the ability to install multiple versions of code so that an application can choose which version of the common language runtime or of a component it uses. Subsequent installations of other versions of the runtime, an application, or a component will not affect applications already installed.

In ASP.NET, applications are said to be running side by side when they are installed on the same computer, but use different versions of the .NET Framework. The following topic describes how to configure ASP.NET applications for side-by-side execution and provides detailed steps to:

• Maintain your Web application�s mapping to .NET Framework version 1.0 during installation
• Map a Web application to a specific version of the .NET Framework
• Find the version of the .NET Framework that a Web site is using

Traditionally, when a component or application is updated on a computer, the older version is removed and replaced with the newer version. If the new version is not compatible with the previous version, this usually breaks other applications that use the component or application. The .NET Framework provides support for side-by-side execution, which allows multiple versions of an assembly or application to be installed on the same computer at the same time. Because multiple versions can be installed simultaneously, managed applications can select which version to use without affecting applications that use a different version.

Reference: http://www.asp.net/learn/whitepapers/side-by-side-with-1.0/