It is possible to reconstruct the geometry of Vermeer’s interiors, for those paintings where parts of the tiled floors are visible. This can be done by reversing - in effect - the conventional process of setting up perspective drawings. The actual dimensions of the room or rooms, and the shapes and sizes of pieces of furniture, can all be calculated with precision. This is because Vermeer depicts a number of recognisable real objects — chairs, Delftware tiles, musical instruments, wall-maps and paintings by other artists — which all survive in museums today. It turns out that in ten paintings Vermeer does indeed depict a single room whose dimensions are broadly consistent throughout. He reproduces the furniture and maps at something very close to their known sizes, allowing for perspective diminution with distance. In one painting, ‘The Music Lesson’, Vermeer includes a mirror which reflects his own vantage-point and a part of the back wall of the room, behind him, not otherwise visible in any picture. This evidence provides a measurement for the length of the room. The dimension is compatible with the width in plan of both of the houses in which it is believed that Vermeer may have had studios: the inn ‘Mechelen’ on the Market Place owned by his family, and the house on the Oude Langendijk owned by his mother-in-law Maria Thins.