A bilingual is a person who speaks two languages, right? Well, it's a bit more complicated than that. What about someone who understands a second language well but cannot speak or write it? Or someone who speaks one language fluently at home but, for example, has difficulty using it at work or for academic purposes? What level of proficiency do you have to attain in order to legitimately call yourself bilingual?

In fact, linguists tell us, people who have "perfect" fluency in two languages are the exception rather than the rule. The vast majority of bilinguals do not have an equal command of their two languages: one is more fluent than the other, interferes with the other, imposes its accent on the other, or simply is the preferred language in certain situations.

Proficiency in a language involves a number of different abilities and skills: understanding, speaking, reading and writing as well as pronunciation, grammar, vocabulary and what linguists call pragmatics, that is, the set of mostly unconscious social rules that constrain the way we speak. For example, the choice between tú or Vd in Spanish, or the importance of always saying please and thank you in British English. In addition, people may be bilingual to various degrees depending on such factors as circumstance of acquisition, opportunities for use of the other language, aptitude and motivation.

It's far more helpful to think of ability with more than one language as a continuum. At one end is native-like mastery of two or more languages, at the other minimal competence in a second language and/or limitations in the use of both languages. Most bilinguals will be at different points in between, and our children are no exception. Even when both parents are English speakers, living here in Spain the kids will inevitably develop asymmetrical skills. in English and Spanish (and, depending on location, in a co-official language like Catalan, too).

If they go to a local Spanish school, of course we'll want to teach them reading and writing skills in English or make arrangements for this. Even so, some of their speaking and listening skills will lag behind: their English vocabulary will be limited simply by the fact of living in a Spanish environment. On visits home, they may well find themselves out of date with the latest slang (and so indeed may we). They may have some (initial) difficulty in handling school subjects in English and understanding a wide variety of regional accents. Today, with the internet, satellite TV and other wizardry, it's easier than ever for us to reinforce our children's English. But quaintness may be par for the course. Ten-year-old trilingual Eve has always enjoyed a nourishing diet of BBC videos. "Her English sounds a bit theatrical, as if she's declaiming," says her dad. "She sounds like Dame Edith Evans."