In this Vermeer, a woman is intently writing while her servant apprehensively looks out the window, waiting to deliver her mistress' letter. There's a crumpled paper on the floor which people used to think was a discarded first draft until someone cleaned the canvas and found a dab of red next to it--sealing wax. (Click on the picture for a better look.) The letter angered her, but now she's collected herself and is penning her response. On the wall is a painting depicting Pharaoh's daughter, with her maidservants, finding Moses among the bulrushes. One of them, by tradition Moses' sister, stands behind the princess exactly as the servant stands behind the writer.
Symbolically, there are several possibilities. Perhaps a negotiant is testing the woman's mettle, which will prove regal. Perhaps the mistress is pregnant and her suitor has rejected her, but she's not about to cast the babe upon the waters. Perhaps it is the servant who is in distress and the woman has resolved to defend her. In any case, there's a pact between servant and mistress, and much is at stake.
For me, this painting is about the reach and strength of women. Of the many ladies Vermeer painted in domestic scenes, quite a few had pens in their hands. Writing is an expression of will, enabling women to spread their influence past the confines of their homes. I have no doubt that Vermeer wants us to heed what they have to say. .