  Let $C$ be an algebraic curve of genus $g\ge2$. A coherent system on $C$
consists of a pair $(E,V)$, where $E$ is an algebraic vector bundle over $C$ of
rank $n$ and degree $d$ and $V$ is a subspace of dimension $k$ of the space of
sections of $E$. The stability of the coherent system depends on a parameter
$\alpha$. We study the geometry of the moduli space of coherent systems for
$0<d\le2n$. We show that these spaces are irreducible whenever they are
non-empty and obtain necessary and sufficient conditions for non-emptiness.
