A proportion is an equation showing the equivalence of two ratios, or fractions, with different numerators and denominators. a/b = c/d or a:b=c:d The first and last numbers in the proportion are called the extremes, while the other two numbers are called the means. ↓ a0.75em extremes means : ↑ b= ↑ c:↓ d

In a proportion, by the Cross Products Property, the product of the extremes is equal to the product of the means.

As an example of proportionality, consider slices of pizza. Depending on the number of times it has been sliced, the same amount of pizza could be cut into 1, 2, or 4 pieces. In this case, one-third of a pizza is the same amount of pizza as two-sixths or four-twelfths. If the simplified forms of two fractions are equal, then they are said to be proportional. For example, one-third is proportional to two-sixths and four-twelfths.

Note that proportions are often used in geometric concepts such as the Triangle Proportionality Theorem or when determining if two figures are similar.