A definition of bilingualism can include speakers of different languages as well as those who speak several dialects or several sub-varieties of dialects in the same language. Most speakers are able to practice code-switching, whether it is from language to language or dialect to dialect, and the processes involved in such a capability may be the same in both cases. Children learning two languages simultaneously demonstrate processes at work in code-switching and language development. A speaker may have a common core of rules for his codes; the rules specific to a particular code are tagged as such through a process of differentiation. Such a system is evident in the learning processes of young children developing the command of yes/no questions in both English and French. (VM)