Before revision: Overalying this but in the same gimbal or suspension is a transparent plate which can be rotated around the same axis as the compass card but has sufficient friction (or a mechanical clamp) to stay fixed relative to the gimbal system once set to a course.
After revision: Overlaying this but in the same gimbal or suspension is a transparent plate which can be rotated around the same axis as the compass card but has sufficient friction (or a mechanical clamp) to stay fixed relative to the gimbal system once set to a course.