In contrast, the dual language system hypothesis states that bilinguals have a separate system for the L1 and L2 which they learn right from the start, so both languages can be acquired simultaneously. Research on vocabulary development have generally provided strong support for this theory. Monolingual children in early language development learn one term for each concept, so does a bilingual child, just that the bilingual child does so for both L1 and L2, and hence they know two language terms of the same concept that has similar meaning, which is also known as translational equivalents. The awareness of synonyms do not appear till a much later age. For example, they know that both 'two' in English and 'dos' in Spanish refer to the numerical number '2'.