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. Measurement interpretation distinguishes resolution from closeness to an accepted reference. The ruler’s endpoint difference estimates length even when the origin is shifted, provided both readings use the same calibrated scale. Real uncertainty includes additional contributions. The ideal diagram isolates scale divisions and endpoint subtraction, while physical measurements also involve calibration and alignment uncertainty.