Protected

actual_use_case.rb

# A good use case would be allowing an object to compare itself to another instance
class Country
  attrs :name, :coordinates

  def overlapping?(other)
    self.coordinates_as_array.include?(other.coordinates_as_array) 
  end

  protected

  def coordinates_as_array
    [x_coord, y_coord]
  end
end

usa = Country.new('USA', 31.123, 28.678)
canada = Country.new('Canada', 29.321, 17.456)
# USA is allowed to call protected method 'coordinates_as_array' on Canada.
usa.overlapping?(canada) #=> false

unexpected_proof.rb

# The following usage demonstrates that `private` does not limit methods
# to subclasses any more than protected does.
#
class TestClass
  protected
  def a_protected_method
    puts "Superclass Protected Method"
  end
  private
  def a_private_method
    puts "Superclass Private Method"
  end
end

class TestSubclass < TestClass
  def call_protected_method_on_superclass
    a_protected_method
  end
  def call_private_method_on_superclass
    a_private_method
  end
end

TestSubclass.new.call_protected_method_on_superclass
# The following call is expected to raise an exception, but doesn't
TestSubclass.new.call_private_method_on_superclass