A navigational device or jig is provided for accurately guiding the drilling of holes for screws or guidewires for spinal surgery. The jig has a three-point attachment to a vertebra with a jaw that contacts the pars interarticularis and a clip that contacts or attaches to the spinous process. A drill guide support arm moveable laterally with three axes of rotation is provided. The drill guide is a channel that directs a drill bit to a desired location and trajectory. The holes created with the drill bit in a vertebra are used for the insertion of screws or guidewires for any of various procedures requiring screws implanted in a vertebra. In particular, the inventive jig is useful for the precise placement and control of drilled holes for cortical bone trajectory screw placement.