Access Modifiers (public, private, ...)

By default;

classes: internal by default. (can be internal or public, can NOT be private, protected or protected internal)
interfaces & enumerations: always public, no access modifier needed or allowed.
namespaces: always public
Struct members: private by default. (can be public, private, internal)

public: accessible from instance & derived classes
private: NOT accessible from instance nor derived classes
protected: NOT accessible from instance but accessible from derived classes
internal: accessible from instance & derived classes WITHIN THE SAME ASSEMBLY ONLY.
protected internal: NOT accessible from instance but accessible from derived classes of the same assembly.

So, if a member is marked as protected, then it is accessible from derived classes of the SAME assembly or other assemblies.