Measurements are specified with a desired quantity and a tolerance. For example, we may specify a length to be measured on an X-ray photograph to have a dimension of 2.000.05 cm, which means that the length may measure from 1.95 to 2.05 cm and still be acceptable. In addition to using the correct instrument to measure the desired precision, the length itself must be reasonable for measurement. It would be poor practice to measure a rough cast iron surface to a precise dimension. The precision of measurement depends not only on the precision of the instrument but also on how precise the length of the object measured is. Fractions sometimes require decimal conversions for correct precision. Significant figures should be observed and rounding should be used for a correct practical precision. The following example illustrates these considerations where a 4-3/8 inch long rod is being measured with an inexpensive ruler. Note that the second and third entries(*) in the table are rounded to 0.05 in. because any desired precision greater than this would require an instrument better than an inexpensive ruler