Probably the water in the gutter runs out into a ditch or canal, in front of the picture. Without the diagonal of the gutter the picture would have less depth. And with the gutter Vermeer draws the viewer's attention to the woman in the passageway. Clearly, the artist found this through-view important; as an overpainting shows. Originally there was also a second woman in the gateway, but because her figure blocked the view through the passageway, Vermeer painted her out. Vermeer would have seen similar through-views by his colleague and townsman, Pieter de Hooch. However, only rarely did De Hooch employ this technique as successfully as Vermeer has here.