Before the adoption of the International System of Units ( SI) , there are many different units. Today, the SI units are the adopted system of units in the scientific community because all SI units are based on seven fundamental units, prefixes, and the power of 10. I will explain this in the later video because we will focus on the different SI base units and units that are derived from the SI base units in this video. Again, all SI units are based on the seven fundamental SI units called SI base units. Currently, Liter is considered as one of the SI base units, so there are eight SI base units. The SI base units are like the 26 letters of the English alphabet. We combine the letters to make different words that have different meanings. Unlike the English alphabet, the SI units are based on seven fundamental units we called base units. Each base unit is like a letter in the alphabet as each base unit is used to measure a specific physical quantity. According to the SI units system, the letters are: meter, kilogram, Kelvin, Liter, mole, second, Coulomb, and candela. Different SI base units are used to measure different physical quantity, and they are represented by different symbol in a mathematical equation. Also, different SI base units are abbreviated to make recording easier. We mention before, that different SI bases are like letters in the alphabets, and they can combine to make new units. We call the new units that are made from a combination of SI units a derived SI unit. Derived units are made by multiplying and dividing SI units.The word “derived” means is coming from something. If we look at the unit dimensions of a derived unit, the derived units are made up of multiple on or more SI base units. For example, density is made the SI base unit of mass by divided by the SI base unit volume. One derived SI units of density is kilogram per Liter ( kg/L). In this example, different SI Base units are divided by each other. Another example of a derived SI unit for measuring area is meter square (m2). To find the area, we multiply measurement of length and the measurement of width. In this example, the same SI unit is multiplying to each other, giving us meter square (m2).