These are two rulers perpendicular to each other, also known as a square measure!
You can use this 90 degree ruler in many ways, such as drawing or measuring something.
Everything is labelled with an extra layer, you can add a filament change to the last layer to better see the text :D
I made this to help me measure something I couldn't measure with just a 15 cm ruler.
TAKE NOTE! the ruler itself isn't 15cm by 20cm, but it is the amount it measures.
The ruler's maximum Length is 20cm+0.5cm+2.5cm = 23 cm
The ruler's maximum Width is 15cm+0.5cm+2.5cm = 18 cm
Please make sure it fits you're build plate! (It fits the BambuLab P1 and X1 series printers)