Every measurement involves errors, so the result must be reported in a way that indicates its precision. A reported result normally includes all the digits known reliably plus the first uncertain digit; together these are called significant digits or significant figures. For example, a pendulum period of 1.62 s has three significant figures because 1 and 6 are certain while 2 is uncertain. Reporting more digits than the significant ones is superfluous and misleading because it suggests a precision that does not exist. Significant figures depend on the least count of the measuring instrument, and a change of units does not change their number: 2.308 cm, 0.02308 m, 23.08 mm and 23080 µm all have four. The rules are as follows. All non-zero digits are significant. Zeros between two non-zero digits are significant. For a number less than 1, zeros to the right of the decimal point but left of the first non-zero digit are not significant. Trailing zeros in a number without a decimal point are not significant, whereas trailing zeros in a number with a decimal point are significant. To remove ambiguity, report measurements in scientific notation \(a\times10^{b}\); all digits in \(a\) are significant. Exact numbers, like the 2 in \(s = 2\pi r\), have infinite significant figures. The exponent \(b\) of 10 after rounding \(a\) is the order of magnitude.