1. 3 Physical quantity unit(单位、量纲): what was measured Two major unit systems: United States Customary System( USCs, British System of Units) Systeme International(si, metric system)1.3 Physical quantity unit (单位、量纲): what was measured. Two major unit systems: United States Customary System ( USCS, British System of Units) Systeme International (SI, metric system)