1 master page can refer more
than 1 master page called nested master page.Each child master page contain their own componentized content placeholders that are mapped to content placeholders on
the parent master page.
For example, Different web site content partners can define their own child master pages that reference the child master
and that is define in partner's content.So the main master page can use as archive of child masters.
The main master page like this:
<%@
Master Language="C#" MasterPageFile="~/Base.master" AutoEventWireup="false" CodeFile="Derive.master.cs" Inherits="Derive" %>
<asp:Content ID="Content1"
ContentPlaceHolderID="head"
Runat="Server">
</asp:Content>