Reliability of measuring tools?

[LesForgue]

I got a big aluminum square from Menard's or Home Depot (forget which), and while I was working on drawing panel cut lines onto my plywood, I realized that so and so many inches on that thing does not match that measurement on my old steel square and several other rulers.
I had a bit of erasing to do becasue of the inconsistency.

If you can't trust a ruler, well, I don't know,
Anybody else experienced such?

Or, Maybe the square that seems off is actually a state of the art instrument, adjusted for the curvature of the earth?

[tx river rat]

An old timer taught me the first thing you did with a new framing
was to square it, he did the squaring with a center punch .
not sure about the markings maybe its in tenths or mm
Ron

[neon14]

Had that problem before. I have came to the conclusion that nothing is square.

[lilmoe]

I was taught to always check a new square by putting against a straight edge, mark along the edge, then flop it side to side, and check your mark. Most of them will not be perfect, but can be adjusted with a punch to be the same. A level should also be checked, but sometomes you just have to go with It looks right.