1. Metric system: The most widely used system for measuring length, mass, and volume. Common units include:

* Length: Meter (m), centimeter (cm), kilometer (km)

* Mass: Gram (g), kilogram (kg)

* Volume: Liter (L), milliliter (mL)

2. Imperial system: Still used in some countries, particularly the United States. Common units include:

* Length: Inch (in), foot (ft), yard (yd), mile (mi)

* Mass: Pound (lb), ounce (oz)

* Volume: Gallon (gal), quart (qt), pint (pt)