In the early stages of bilingual development, it tends to be more helpful if the child consistently speaks only one language with a particular person. For the young child, it is often confusing when anything different from 'one language per person' is apparent. Imagine the situation where a child speaks French to the father who answers in English. The child attempts a reply in English and then the father switches to French. The child may become insecure and muddled. The younger child requires some security and stability when speaking and listening. It is also valuable to set out clear language boundaries and language compartmentalization.

Once the child has achieved reasonable language separation, it is not uncommon that older children will use both languages with the same person. In a one to one situation, the older child's use of languages with a particular person will usually be stable and relatively invariant. Once other people enter the conversation, the older child may switch languages. Children are amazingly adroit at knowing when to switch languages. Partly for this reason, bilinguals seem to have some degree of social sensitivity that monolinguals do not have to have in a language situation.