Classes and Inheritance - Illustrated C# 2005 - page 163

Protected Internal Member Accessibility
Members marked protected internal are visible to all the classes that inherit from the class,
and also to all classes inside the assembly, as shown in Figure 7-20. Notice that the set of classes
allowed access is the combined set of classes allowed by the protected modifier plus the set of
classes allowed by the internal modifier.
Figure 7-20. Protected internal member visibility