Accessing undefined/unitialized global or instance variables (like: @i) is not an error and nil will be returned. With -w a warning will be emitted. The weird class variables must be initialized. Local variables (lexically scoped) must be defined and are default-initialized to nil. (Global and class variables are usually not used.)

another example: > a=a => nil

but then:

> a=b NameError: undefined local variable or method `b' for main:Object