common underlying proficiency (CUP)
Research supports this model of how bilingual knowledge and ability is stored in the brain. Rather than having two separate areas for different languages, the two languages are stored together and the knowledge is linked and can interact. The two languages are kept separate only at the surface level, where they are used to speak, read, write, and so on. They are like an iceberg, with a single big mass under the surface and two small peaks showing above it. This way of understanding bilingualism is also sometimes referred to as the iceberg analogy.
