Resumen: We find conditions on a function space that ensure that it behaves as an -space in the sense that any unconditional basis of a complemented subspace of either is equivalent to the unit vector system of or has a subbasis equivalent to a disjointly supported basic sequence. This dichotomy allows us to classify the symmetric basic sequences of . Several applications to Orlicz function spaces are provided. Idioma: Inglés DOI: 10.1007/s11117-024-01093-w Año: 2024 Publicado en: POSITIVITY 29, 1 (2024), 1-36 ISSN: 1385-1292 Financiación: info:eu-repo/grantAgreement/ES/DGA/E48-23R Financiación: info:eu-repo/grantAgreement/ES/MCINN/PID2022-137294NB-I00 Tipo y forma: Artículo (Versión definitiva) Área (Departamento): Área Análisis Matemático (Dpto. Matemáticas)