This strange-looking cold chisel has a flared profile that’s wider at the head and gets smaller as you go back. This allows you to make grooves, keyways, or slots without the chisel binding on the sides of the slot.
KD tools, one manufacturer among many, sells two sizes of cape chisels. The first cuts a 1/4″ slot and measures 5″ long with a 3/8″ hex shaft and the other cuts a 3/8″ slot and measures 6″ long with a 1/2″ hex shaft.
The 1/4″ chisel runs starts at $6 and the 3/8″ chisel starts at $10.