The bilingual identical twins from the FerFuLice corpus are Simon and Leo. They were born into a middle-class family in Spain. The father, Ivo, is a native speaker of Peninsular Spanish and the mother, Melanie, is a native speaker of American English. The parents have kept a strict separation of both languages so that each parent addresses the children in their own mother tongue (Grammont rule (Grammont 1902); one person, one language (Ronjat 1913)). The parents generally communicate in Spanish with each other, except in summers when the family travels to the United States for approximately two months or when a monolingual English speaker is present. Therefore, we are dealing with bilingual first language acquisition (Spanish and English) in a monolingual social context (Spanish), a type of bilingualism that is referred to as individual bilingualism (Bhatia & Ritchie 2004).

During the first year, the mother was the primary caretaker of the twins. The father was present all day on weekends and less on weekdays. At age 1;10, the twins started going to day care, where the language of the staff and other children was Spanish. Apart from the mother, additional contact with English was provided by occasional visits by the maternal grandparents and during the two-month visits to the United States every summer.