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This paper questions how ethnographic fieldwork (in sociolinguistics) may contribute to theoretical generalization despite the admitted singularity of its data. On the basis of epistemological work of Passeron (2006) which conceives humanities and social sciences as « historical sciences », the author draws upon his ethnographic research among adolescents in schools to show how such an approach allows to aim generalization and to discuss the relevance of a concept –  crossing (Rampton 2005) – regularly used in “youth language” research.