This tool is used to help plumb / level something.  In conjunction with a laser I use this tool to check that a bunch of metal cabinets are all flat / plumb to the wall I'm installing them on.

These print in halves to make the color change easy and to add strength to the print so it does not snap in half easily. Print (2) and affix together.

I use 10mm x 2mm magnets in the ends.  Everything is epoxied together for strength.