  Our aim in this article is to contribute to the study of the structure of
subsymmetric basic sequences in Banach spaces (even, more generally, in
quasi-Banach spaces). For that we introduce the notion of positioning and
develop new tools which lead to a dichotomy theorem that holds for general
spaces with subsymmetric bases. As an illustration of how to use this dichotomy
theorem we obtain the classification of all subsymmetric sequences in certain
types of spaces. To be more specific, we show that Garling sequence spaces have
a unique symmetric basic sequence but no symmetric basis and that these spaces
have a continuum of subsymmetric basic sequences.
