I consider myself something of a connoisseur of antique school slates at this point, having bought and sold a whole lot of them, and looked at many many more. This one, found in southwest New Hampshire, sold me for the unusual narrowness of the frame, and for its unusual and very sweet notched corner joints. Also, there is a layer of what appears to be iron in between the two layers of the frame. Someone more knowledgable than me can surely understand the reasoning behind the construction, which I've never seen before on a slate--whatever the case, part of what I love so much about these old handmade ones is their individuality, much deepened over time through use; each one feeling personal, and specific, like a portrait.  

11 7/8" x 7 3/4" and in good antique condition, warm patina to the wood with darkening around the far edges of the perimeter from the iron between the two layers of wood--which is just barely visible. That iron makes it heavier than one might expect. Sturdy and sound.