This is a variable width picture frame spacer & level.
You can use it to evenly space picture frames and create those beautiful picture walls.
The sizes range from 2cm to 5cm, and all the different spacers can be assembled into a single tool for easy usage and storage. 

You can optionally insert 3 mini levels into the slots to turn this into a dual axis level.
You'll need 3 rounded 40cm x 9.5cm mini levels (you can find them here: https://s.click.aliexpress.com/e/_DBPUqBD)