Along with the gorgeous resin material, there's a massive ink window (turns out "fenestro" means "window" in Esperanto) that shows you a clear view of how much ink is remaining in the converter cartridge or in the ink reservoir if you've chosen to fill the pen as an eye-dropper, which is completely supported by the Fenestro. There's a beefy o-ring above the threads of the section that connect to the ink reservoir, so you don't have to worry about any ink leaking out if you choose to fill it with an eyedropper. According to JetPens, you can fill 3.6 ml of ink into the pen this way!
The pen is as versatile as it is beautiful. It can be used with nearly any standard international cartridge or converter, or you can fill the barrel directly with up to 3.6 ml of ink using the included plastic eyedropper. The durable resin barrel is acid resistant and alkali resistant, making it safe to fill with any fountain pen ink.
Moving on to the size of the pen, it's not exactly a pocket pen. It's fairly large at just under 6 inches when capped, and a whopping 7 inches when posted. I haven't posted this pen at all because I find the uncapped length (5.3 inches) to be perfect for me.