  The bricks over preprojective algebras of type A are known to be in bijection
with certain combinatorial objects called "arcs". In this paper, we show how
one can use arcs to compute bases for the Hom-spaces and first extension spaces
between bricks. We then use this description to classify the "weak exceptional
sequences" over these algebras. Finally, we explain how our result relates to a
similar combinatorial model for the exceptional sequences over hereditary
algebras of type A.
