Significant digits, also called significant figures, are not just for measurement. They are used to define certain calculations and follow certain rules. For instance, the digits one through nine are always considered significant digits; the number 66 has two significant digits in a calculation, while 66.6 has three significant digits. In division, multiplication, trigonometric functions, and so on, the number of significant digits in an answer should equal the least number of significant digits in the numbers being divided, multiplied, and so on. With zeros, it’s a bit more complicated. For example, zeros before other numbers are not significant digits, such as 0.066 has only two significant digits; and zeros within other numbers are significant digits, such as 6006 has four significant digits. For larger numbers, it’s easier to see significant digits if you use scientific notation. For example, 2.2 × 10^{3} has two significant digits; 2.20 × 10^{3} has three significant digits (zeros placed after the decimal point become significant digits).