Master page is a template page that can be used to create a consistent layout for your application. First you create a master page to define the look & feel of the application and then you create the content pages that contains the content.
You can attach these content pages to the master page at the following three levels:
By setting master page programmatically or thru web.config you may not get the Visual Studio master page design time features.
Deepak Verma SDET | Visual Web Developer