GAMOV-SPHERE

class Foo 
   @a = 123	   # Class instance variable (1)

   @@a = 456   # Class variable (2)

   def foo 

         p @a     # => nil not 123 or 456  - Ordinary instance/object variable  (3)

   end 

end

• (3) is an ordinary instance variable (which, not having been initialized, has a value of nil), it belongs to an instance of class Foo.
• (2) is a ordinary class variable unique and accessible to all object instances of class Foo
• (1) belongs to the class object Foo, which is an instance of Class class.

The last one (1) is quite strange for someone coming from languages like Java. A good rule of thumb to know to which object a variable is attached to is:

The first mention of an @-prefixed variable creates the instance variable in the current object, self.

Therefore, since everything is an object in Ruby, if you declare a @-variable when self is the class, the variable will be attached to it.