ASP.NET - NESTING MASTER PAGES

Example:
You have only content with such ID. Put another placeholder inside of content with ID="Content2" and then connect with the page content.
Master Page

 ID="bodyContent" runat="server">

Nested Master Page

 ID="Content2" ContentPlaceHolderID="bodyContent" runat="server">
     ID="nestedContent" runat="server">
    
 

Nested Master's Content Page

 runat="server" ID="myContent" ContentPlaceHolderID="nestedContent">