ASP.NET Master Pages

<%@ Master ... %>

<%-- HTML code --%>
<asp:ContentPlaceHolder id="plhMainContent" runat="server" />
<%-- HTML code --%>

<%@ Page ... master="~/MasterPage.master" ... %>

<asp:Content ID="ContentIdentifier" ContentPlaceholderid="plhMainContent" runat="server">
    <%-- More HTML here --%>
    <%-- Insert web controls here --%>
</asp:content>