Chain Checker Instructions - Translated from German.

Two new measuring devices make the elongation of the chain visible and make it easier to decide whether the link strand needs to be replaced.

With bicycles with derailleur gears, the existing technology cannot avoid the fact that chains are wearing parts: after 800 to 5,000 km many have become longer due to abrasion on the bolts and are no longer able to grip the gears properly.  Sooner or later this will lead to severe slipping.  However, the sprockets are already ruined by the chain links that are too long.
It is not good advice to always replace the sprocket and chain together.  Economical cyclists change the chain before the sprockets are destroyed.  Unfortunately you can see it
Do not chain this wear point, so you have to measure the length.  The author always did this using the Park Tool spoke gauge, which also has inches marked on the side.  A chain link is 1/2" long, and according to the old rule of thumb, the chain went into the trash when it became 1/8" longer than the 12" on the gauge
The same thing is now much easier and quicker with this.

Chain checker by Stephan Pochert.  You place an aluminum rod with two pins on the chain.  One of the pins sits on an eccentric disc, which you turn until both pins press tightly against the chain rollers.  You can easily read the elongation of the chain using a scale on the eccentric disc.  Additionally, the scale shows “red” if the chain is longer than Hyperglide or aluminum chains