Abstract: Stereotaxic Arc Systems are common instruments in neurosurgery for accurately directing a probe into the head and brain. Among the many types of arc concepts for such instruments is the target-centered arc. This involves an arc system which provides spherical radii from a multiplicity of directions depending on the rotation angles of the arc. All of the radii converge to the target point, thus the name target-centered arc. Prior to this invention, all such systems with two trunion bearings have an axis of arc rotation which has a fixed orientation relative to the patient's head. This causes a limitation in directions of approaches to desired targets. The present invention involves a new concept of target-centered arc, in which the access of rotation of the two trunions and their connected arc itself can be varied in orientation, leading to an unprecedented wide range of approaches to any anatomical target and great versatility of the arc system in surgery.