Nesting Master Pages

There are times where you may want to provide overall branding and appearance for your Web site, but at the same time provide sub-sites whose appearance must be consistent. To accomplish this, you need to nest one master page within another page, a process called "Nesting Master Pages". Nesting support is built-in, which means you don't have to do anything special to reap the advantages of nested master pages; a content page that uses a sub-master page will automatically receive content from all the master pages in the hierarchy.

Using the master pages architecture, you could implement a consistent look and feel across all the developer centers. For example, you can have the main MSDN page derive from a root master page. Each developer center can have its own master page, each of which uses the root master page as its master. This means a content page in any of the developer centers will inherit the root master page look and feel settings that provide overall MSDN branding, and will also inherit the custom look and feel specific to that developer center. This nested approach provides each developer center with the freedom to develop customized content while maintaining a consistent overall branding for the entire site.

ref:devx