How much can a ruler reading tell you?

To measure length, read both ends of an object and subtract the start from the end. The end reading alone is not the length when the start is away from zero. Finer ruler marks make smaller differences easier to read. They do not make the object longer. Resolution describes the smallest scale difference an instrument can distinguish. A millimetre scale provides finer divisions than a centimetre scale, but alignment and calibration still matter. Read an object’s two endpoint positions and subtract them. Reporting extra digits without supporting scale information does not create a more trustworthy measurement.