Method names in Ruby may contain upper-case and lower-case letters, numbers, underscores _ and the punctation signs !, ?, =. A method name can't begin with a number, and the characters !, ? and = can only appear at the end. Non-ASCII characters can be used in a method name, but this can lead to very confusing situations and should not be common practice. It's good practice, while not mandatory, to start the method name with a lower-case character, because names that start with capital letters are constants in Ruby. It's still possible to use a constant name for a method, but you won't be able to invoke it without parentheses, because the interpeter will look-up for the name as a constant: