The set of rational numbers ℚ = {m/n} consists of numbers that can be expressed as ordinary fractions (for example, 4/3). In decimal representation, they can be finite or repeating decimals, such as 4/3 = 1.333... The term "rational" comes from the Latin word "quotient," which furthermore developed in English. It's interesting enough that the set ℚ is also countable (equivalent in cardinality to the set of natural numbers ℕ).

The sets [U:2115], [U:2124], [U:211A], [U:211D], [U:2102] are often written in bold font (for example, in the works of Nicolas Bourbaki) or as double-struck letters on blackboards and manuscripts. The usage of double-struck font in printed texts was introduced by American mathematicians and became widespread with the famous textbook on complex analysis by Gunning and Rossi, published in 1965.