"The uncertainty should be rounded off to one or two significant figures. If the leading figure in the uncertainty is a 1, we use two significant figures, otherwise we use one significant figure. Then the answer should be rounded to match." "Here’s a rule of thumb you can rely on: round the uncertainty to one significant figure. Then round the answer to match the decimal place of the uncertainty. One exception to the rule of thumb: If rounding the uncertainty to one significant figure would cause that figure to be a 1, then you keep the next digit as well." Both the quotes are taken from leading universities such as Harvard, regarding the number of significant figures to keep (in uncertainties in Physics), and they both say the same thing. So my question is: Why is a leading figure of 1 so special in uncertainties (in physics) that the said uncertainty deserves two significant figures, as opposed to just one sig fig?