Add the following declaration to content pages to allow easy access to master page public properties and controls.
<%@ MasterType VirtualPath="~/MasterPageName.master"%>
This will allow you to reference a public property or control by making a call like “me.master.PropertyName”. Extra code to cast would be needed if you did not specify the MasterType.

MasterType can also be set when using nested master pages, but you will not be able to reference the top master's controls without creating a public property for each.
Setup nested master pages as shown below.

<%@ Master Language="VB" CodeFile="MasterPageInner.master.vb" Inherits="MasterPageInner" MasterPageFile="~/MasterPageOuter.master"%>
<%@ MasterType VirtualPath="~/MasterPageOuter.master"%>

<asp:Content ContentPlaceHolderID="ContentOuter" runat=server>
    <asp:contentplaceholder id="ContentInner" runat="server">
    </asp:contentplaceholder>
</asp:Content>

Content page:

<%@ Page Language="VB" MasterPageFile="~/MasterPageInner.master" AutoEventWireup="false" CodeFile="MasterTypeTest.aspx.vb" Inherits="MasterTypeTest" title="Untitled Page" %>
<%@ MasterType VirtualPath="~/MasterPageInner.master" %>
<%@ Reference VirtualPath="~/MasterPageOuter.master" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentInner" Runat="Server">
    <asp:Literal ID="LitTest" runat="server"></asp:Literal>
</asp:Content>