How accurate are rulers?

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Yes... but that's no good if the ruler is inconsistent with itself - i.e. if the error is not uniformly distributed on the rule.
Also there are lots of times where dimension A is measured with one device (say a tape for length), dimension B uses another (e.g. a ruler for width) and C for a third (for example calipers for thickness)[/quote]

Well. Not really a problem either is it?

If you use the same ruler or measuring device for all the measurements on a given project and always measure from zero with the numbers increasing toward the end of the ruler then all your measurements will be correct, relevant to each other, or will they? I've confused myself now.[/quote]
You could have a ruler that measures the first foot as 12.0", then the second foot as 12.1" as an example. In that case, two foot long pieces as measured would not be the same as a two foot piece.

If you wanted everything accurate relatively you'd need to take a measurement then scale everything off that (by halving / doubing, etc.)[/quote]

Yeah I realise that but who cares?

If you use the same tool for a project and measure from zero up your error will always be the same. So the bits of wood you cut to fit the hole you measure will always fit.
 
Rich C":j7uvtmte said:
....
If you wanted everything accurate relatively you'd need to take a measurement then scale everything off that (by halving / doubing, etc.)
This is what dividers are for. Very precise engineering is possible with dividers, involving no tape/scale measurement at all
 
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