Editing Significant figures (section)

== Estimating an extra digit ==
When using a ruler, initially use the smallest mark as the first estimated digit. For example, if a ruler's smallest mark is 0.1&nbsp;cm, and 4.5&nbsp;cm is read, then it is 4.5 (±0.1&nbsp;cm) or 4.4&nbsp;cm to 4.6&nbsp;cm as to the smallest mark interval. However, in practice a measurement can usually be estimated by eye to closer than the interval between the ruler's smallest mark, e.g. in the above case it might be estimated as between 4.51&nbsp;cm and 4.53&nbsp;cm.<ref name="UMmeasurements">{{cite web
|title=Measurements
|url=http://slc.umd.umich.edu/slconline/SIGF/page7.html
|url-status=dead
|archive-url=https://web.archive.org/web/20170709090746/https://slc.umd.umich.edu/slconline/SIGF/page7.html
|archive-date=2017-07-09
|website=slc.umd.umich.edu
|publisher=University of Michigan
|access-date=2017-07-03
|df=dmy
|quote=As a general rule you should attempt to read any scale to one tenth of its smallest division by visual interpolation[example omitted].}}</ref>

It is also possible that the overall length of a ruler may not be accurate to the degree of the smallest mark, and the marks may be imperfectly spaced within each unit. However assuming a normal good quality ruler, it should be possible to estimate tenths between the nearest two marks to achieve an extra decimal place of accuracy.<ref name="Weston">{{cite book|title=Experimental Electrical Testing|url=https://archive.org/details/experimentalele00njgoog|quote=Experimental Electrical Testing..|date=1914|publisher=Weston Electrical Instruments Co.|location=Newark, NJ|page=[https://archive.org/details/experimentalele00njgoog/page/n14 9]|access-date=14 January 2019}}</ref> Failing to do this adds the error in reading the ruler to any error in the calibration of the ruler.