I typically use a router and a template to cut mortises or to enlarge existing mortises for new door hinges. But for small jobs where it doesn’t make sense to cart along a lot of gear, I use a utility knife and a chisel for the same purpose. First I screw the hinge to the door stile in the desired position. Then I score around the edges of the hinge with the knife as shown in the drawing above. With the hinge removed, I chisel the mortise to the thickness of the hinge. Now I can reattach the hinge using the same screw holes.
—Daniel E. Hill, Ill, Griswold, CT
Edited and illustrated by Charles Miller
From Fine Homebuilding #78
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Still waiting for the missing drawings from the first hinge mortise 'tip'.
In the time before routers every carpenter was expected to know this technique. If you have a router but no jig for your hinge you can outline the mortise as described in this tip with knife and chisel, remove most of the wood with the router, and clean up with the chisel.
That's how I've been doing it for years. Good tip.