Cummins'（1980a）Common Underlying Proficiency model of bilingualism can be pictorially represented in the form of two icebergs（see below）.The two icebergs are separate above the surface.That is，two languages are visibly different in outward conversation.Underneath the surface，the two icebergs are fused such that the two languages do not function separately.Both languages operate through the same central processing system.

Figure 7.11　Common Underlying Proficiency Model of Bilingualism.Source：Colin，B.（1993）

A distinction can thus been made between the Separate Underlying Proficiency（SUP）and Common Underlying Proficiency models of bilingualism（CUP）.The former（SUP）relates to the‘two balloon’idea presented earlier in this chapter；the latter（CUP）relates to the iceberg idea.The Common Underlying Proficiency model of bilingualism may be summarized in six parts：（1）Irrespective of the language in which a person is operating，the thoughts that accompany listening，talking，reading，writing and come from the same central engine.When a person owns two or more languages，there is one integrated source of thought.（2）Bilingualism and multilingualism are possible because people have the capacity to store easily two or more languages.People can also function in two or more languages with relative ease.（3）Information processing skills and educational attainment may be developed through two languages as well as through one language.Cognitive functioning and school achievement may be fed through one monolingual channel or equally successfully through two well developed language channels.Both channels feed the same central processor.（4）The language the child is using in the classroom needs to be sufficiently well developed to be able to process the cognitive challenges of the classroom.（5）Speaking，listening，reading or writing in the first or the second language helps the whole cognitive system to develop.However，if children are made to operate in an insufficiently developed second language（e.g.in a‘submersion’classroom），the system will not function at its best.If children are made to operate in the classroom in a poorly developed second language，the quality and quantity of what they learn from complex curriculum materials and produce in oral and written form may be relatively weak and impoverished.This has been the experience of some Finns in Swedish schools who were forced to operate in Swedish（Skutnabb-Kangas and Toukomaa，1976）.Such children tended to perform poorly in the curriculum in both Finnish and Swedish because both languages were insufficiently developed to cope with given curriculum material.（6）When one or both languages are not functioning fully（e.g.because of an unfavorable attitude to learning through the second language，pressure to replace the home language with the majority language）cognitive functioning and academic performance may be negatively affected.The distinction between Separate Underlying Proficiency（SUP）and Common Underlying Proficiency models of bilingualism（CUP）does not fully sum up the findings from research on cognitive functioning and bilingualism.Therefore this chapter moves on to examining other more sophisticated theories.