.MASTER File Extension
File TypeASP.NET Master Page
What is a MASTER file?
Master Web page or template created with ASP.NET; used for building multiple pages with the same layout; typically includes defined header, navigation, and footer sections as well as editable areas; constructed using HTML, just like a typical Web page.
When editing a Web page based a master page, the non-editable areas are shaded gray while alterable items are clearly shown. Editable areas can be defined by editing the master page directly.
Master pages work in conjunction with content pages, which merge with the master page to produce the output seen by the user.