  We study aisles in the derived category of a hereditary abelian category.
Given an aisle, we associate a sequence of subcategories of the abelian
category by considering the different homologies of the aisle. We then obtain a
sequence, called a narrow sequence.
  We then prove that a narrow sequence in a hereditary abelian category
consists of a nondecreasing sequence of wide subcategories, together with a
tilting torsion class in each of these wide subcategories. Furthermore, there
are relations these torsion classes have to satisfy. These results are
sufficient to recover known classifications of t-structures for smooth
projective curves, and for finitely generated modules over a Dedekind ring.
  In some special cases, including the case of finite dimensional modules over
a finite dimensional hereditary algebra, we can reduce even further,
effectively decoupling the different tilting torsion theories one chooses in
the wide subcategories.
