A simple gauge used to measure the length of a bike chain.
One side is the length of 6 new links, the other shows when the chain has stretched more than 0.5% and needs replacing.
You want your chain to fit one side and be too tight for the other. If it easily fits both then its time to replace your chain!
My other gauge will show if the chains so worn that the cassette and chainrings require replacing too - but if your chain is slack on both sides of this gauge chances are you're past the point of a cheap fix!
Use one or two more shells than usual when printing, use a dark coloured filament as it will get dirty quickly.
Updated to reflect comments received and remixes submitted - should print more easily and work marginally better now!