With a sharp chisel, a skilled carpenter can create a hinge mortise in less time than it takes to set up a router and jig to do it. That said, there are still those who prefer to perform the task with a router, and this presents a problem if installing hinges with corners. Since router bits are round, they can only create mortises with rounded corners.
British tool manufacturer Trend Tool Technology invented their Trend Corner Chisel to remedy this.
It’s a spring-loaded corner-shaped steel cutter suspended within an aluminum block.
You square the body against two adjacent edges of the mortise, deliver a hammer blow, and voila.
The tool runs $31.