
The whole thing measures 12" X 14" X 6". The front of the bag has a main flap that opens with a little clasp on the front. I've heard this clasp can break, but so far, so good. The inside of the front flap has a clear plastic sheet with a little velcro clasp holding it open and closed: It can be used for maps or even sheet music.
Inside, there's a long pouch at the bottom with a velcro closure, just about perfect for my pen roll or smaller pen cases. The pockets up top have held most of my thicker pens, except the Rotring Core. The pocket behind them fits small black notebooks from Moleskine or their clones perfectly.

I'm a little leery about the pens. They only go down into their compartments a few inches, but have held them securely-- note that I only have a Safari, Jinhao and Hero in the picture. None of the pens cost more than $20. Due to the front flap, I could probably reach in for the pens on the side if I absolutely had to. This makes them easy to get to, and I haven't pulled the cap off of one yet: they slide right out. There are also pen/cylindrical object pouches on the side, exposed, for those so inclined.
The main inner pocket has room for a laptop or several notebooks. I have a Rhodia A4 tablet, spiral-bound across the top, along with a Mead Cambridge spiral notebook and my leather journal inside. It can fit more if I need it to. The main inner chamber has a broad, shallow pocket across the back so you can keep a notebook separate from other stuff.
The back of the bag has another compartment, closed over the top by a flap with velcro closures and a zipper closure on the left side and bottom. It opens completely. Doing so reveals more pouches on the inside back area. I have not yet found a use for them. However, the back flap has some hard material and yet another pocket, tight and flat. I'd bet it's designed to hold a writing tablet, and that's just what I've got there.

By the time my Fossil bag came in the mail, this one had won me over. Amazon reviews of the thing have been mixed: Sometimes the clasp breaks, sometimes the stitching goes. But this bag has held up well for the week I've owned it. The compartments have worked well for holding my pen cases and notebooks. I have used its top to write on a time or two.
I really like the way this bag is laid out-- If I had the money, I would send one to Renaissance Leather and request one with better stitching, or maybe more custom pen slots.

