The metric system, or système international d’unités (SI), is a base-10 system first adopted in France.
The basic unit of length in the metric system is the metre.
Metric units are named using prefixes that indicate the relationship to the basic unit (e.g., for length, the prefix centi- indicates there are 100 centimetres in a metre).
Metric units are abbreviated for convenience (e.g., metre is abbreviated with m and centimetre is abbreviated with cm).
Standard measuring tools show iterations of a standard unit from an origin.
The other, older, system of measurement that is also commonly used in the United States and Canada is sometimes called the imperial system and uses “Canadian units.”
“Canadian or imperial” units that are still commonly used include miles, yards, feet, inches, acres, pounds, quarts, pints, and ounces. You may encounter these in hospitals (birth announcements), housing and property (square footage/acreage), cooking and drink (pounds,ounces, quarts, pints), some roads and cars (miles, mileage, miles per hour, gallons), railways, and other contexts where integration with the United States is important.
The perimeter of a polygon is the sum of the lengths of its sides.