Do not trust any "two peg" or "two pin" chain checker, as their reading is only a simplistic approximation of a chain's pitch dimension status.

These devices invariably apply a tensile load on the chain, to two rollers at some distance apart, which would at first appear to be a useful way to evaluate a chain's wear of the type that affects roller-to-roller pitch.
But these devices add in the freeplay of both rollers, freeplay that has no bearing at all on the effective pitch of the chain under load, since the chain running on a sprocket loads all of the rollers in the same direction, not in opposite directions(!) as these tools do.

Further, there are brand-to-brand differences in various chain's roller dimensions and freeplay, even when the chains are new.

Shimano makes a tool with three pegs. It costs a lot, and only gives a "go" or "no-go" reading, which is used only to determine if a chain on a customer's bike should go back out for another service interval, or should be replaced. The Shimano tool uses the third peg to load the other two measuring pegs against their respective rollers in the same direction.

To measure a chain quantitatively, one should use a ruler of known accuracy to measure the distance between pins on the side of the chain, measured over a considerable length of chain (a foot is good), so as to capture the small % of wear in a more visually measurable manner. A new chain spaces the pins at exactly 1/2" apart, and a 12-1/16" measurement across the pins is the wear limit, which is 0.5% stretch.

I use a digital caliper with "inside measure" pins to make two measurements along a chain, and by subtracting one from the other the roller freeplay and roller dimension numbers cancel out. I measure first with the caliper set at 1.2", then again with the caliper set at 5.2". I subtract the 1.2" figure from the 5.2" figure, giving a dimension of exactly 4" plus the actual pitch/stretch wear figure. A result of 4.020" is the wear limit of 0.5% that is prescribed for derailer chain replacement. Replacing the chain at 4.020" usually allows one to install a new chain that still can transmit full power to the worn sprockets, so an extra entire chain lifespan can then also be had from the old sprockets.

I think it is very safe to say that popular chain checkers sold over the counter are a good thing for both tool and chain sales, but not so much for economical operation and timely maintenance.