Bildts is a speech variety used by about 10,000 persons as first or second language in the north Netherlandic province of Fryslân. This paper joins sociological-historical information and linguistic factorisation to explain the emergence of Bildts, a mixed Frisian-Dutch language, which' particular nature involved the mutual adaptation of two speaker groups: a group of mother tongue Frisian speakers who learnt Bildts as a second language and a group of balanced bilingual Frisian-Bildts speakers.