Twybil. Copied from an old tool design, this one is the traditional English pattern and is sometimes also called a Mortising axe. Made of carbon steel with a tang running through the 6" (150mm) beech handle for better leverage.
"A few people have asked me what a twybil actually does; well this is my best explanation:
Traditionally Twybils are used to quickly (and satisfyingly!) make the oval mortises on cleft ash/oak gates. Holes have to be drilled in the normal way and the twybil (or mortising axe) is used to remove the bridge of wood remaining between the holes. This is done with the 'beak' end which simply breaks the bridge away easily, using a leveraging action, because the grain is so short. The mortise side walls are cleaned up with the knife end. All this can of course be done with a chisel and mallet but this way you need just one tool, is quicker and very satisfying to use. To my knowledge there is currently no commercial production of these tools, until now."
- Maurice Pyle, Woodsmith