Help needed with understanding accuracy in measuring tools. I want to go straight :-)

[EddyCurrent]

You almost got me there :-) . But isn't it that a square straight edge has 4 edges that could be used for that against the ones that have only 1 sharp edge?
I believe their purpose is a bit different and definitely i believe i need the square- to compare between rails in all directions / the twist i mean/ .

If I'm reading you correctly you mean this;



But if you angle the straight edge on the left it will bend and not be straight any more.