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. Scale spacing limits what a reading can resolve, while other influences affect accuracy and uncertainty. A displaced starting point is handled by taking the difference between end and start readings. The virtual ruler makes both endpoints explicit. Finer divisions alone cannot reveal an instrument’s zero error, calibration or the observer’s alignment errors.