Divide the unit "1" into several parts, and take one part, which is called decimal unit. That is, the numerator is 1, and the denominator is a positive integer fraction, also called unit fraction, which is called 1/n, and the unit fraction is also called "monomolecular fraction" and "Egyptian fraction".

For example, two-thirds of decimal units are one-third, and one-fifth of decimal units are also one-fifth. The molecular size of a fraction is constant, and the larger the denominator, the smaller the fractional unit; In the case of the same molecular size, the smaller the denominator of a number, the larger its fractional unit. This is because the fractional unit is used to represent the number of shares that a unit is divided into equally.

So the bigger the denominator, the more copies it represents, and the smaller each copy. On the contrary, the smaller the denominator, the smaller the number of copies, and the larger each copy. Therefore, fractional units can help to understand and compare different fractions.