We describe precisely, under generic conditions, the contact ofthe accessibility set at time T with an abnormal direction,first for a single-input affine control system with constraint onthe control, and then as anapplication for a sub-Riemannian system of rank 2. As aconsequence we obtain in sub-Riemannian geometry a newsplitting-up of the sphere near an abnormal minimizer γinto two sectors, bordered by the first Pontryagin's cone alongγ, called the L∞-sector and the L2-sector.Moreover we find again necessary and sufficient conditions ofoptimality of an abnormal trajectory for such systems,for any optimization problem.