The document discusses the rules for determining significant digits in measurements and calculations. It defines 5 rules for identifying significant digits in a number. It then provides 3 rules for performing calculations while maintaining the appropriate number of significant digits: when multiplying and dividing use the least number of significant digits, when adding and subtracting use the least number of decimal places, and round numbers according to whether the preceding digit is odd or even. Several examples of calculations involving significant digits are provided.