three

Three is the natural number that comes after two and before four. In the decimal system it is written as the digit 3. Three is the second prime number and the smallest odd prime, a property that underpins many basic concepts in number theory.

In mathematics, three is a Fibonacci number and a triangular number. It also serves as the base of the ternary numeral system, where numbers are expressed using powers of three. A set with three elements is called a triple, and in geometry a triangle is a figure with three sides and three angles.

In geometry, a triangle’s interior angles sum to 180 degrees. Triangles are foundational in geometry and appear in many constructions and proofs, including concepts of similarity, congruence, and trigonometry. Triangular relationships also arise in various applied fields, such as engineering and computer graphics.

In science and color theory, three appears in several contexts. The triple point is the specific temperature and pressure at which solid, liquid, and gas phases of a substance coexist. In color, three primary colors are defined differently by context: additive light-based models use red, green, and blue, while subtractive models use cyan, magenta, and yellow; traditional pigment primaries are often cited as red, yellow, and blue.

In culture and language, the number three is a common motif. The rule of three is a storytelling principle that emphasizes triads for effect. The prefix tri- denotes three, and many religious and mythological traditions feature triadic deities or triads. In music, a triad is a three-note chord.