These are tools I used in making hollow barley twist candle holders on a lathe and a CNC router.
For more information on their use see this video: https://youtu.be/hHQ5697gIW0
One tool is a story stick to layout the design and the other is a custom set of calipers to set the proper depth of cut.
One side of the story stick can be used for laying out the pencil lines. The other side can be used to check the layout after the part has been roughed out.