triad

A triad is a term used to describe a group of three related elements. The concept appears in several disciplines, where the triad denotes a triple unit with shared properties or a resulting structure built from three parts.

In music theory, a triad is a chord made by stacking two third intervals on a root note, producing three distinct pitch classes. The common triad types are major, minor, diminished, and augmented. A major triad consists of a root, a major third, and a perfect fifth; a minor triad uses a minor third, followed by a perfect fifth; a diminished triad uses a minor third and a diminished fifth; and an augmented triad uses a major third and an augmented fifth. Triads may be in root position or inverted, where notes float in different bass registers to alter voice-leading and harmonic function.

In the history of chemistry, the term triad referred to groups of three elements with similar properties, as observed by Johann Wolfgang Dobereiner in the early 19th century. Examples include lithium–sodium–potassium; chlorine–bromine–iodine; and calcium–strontium–barium. Although useful as an organizing idea, triads were superseded by the development of the periodic table, which accounts for broader patterns across many elements.

In other contexts, the name Triads is used for organized crime groups of Chinese origin and diaspora, often referenced in media. More generally, triad may denote any trio forming a functional unit, such as three related colors, notes, or categories in a system of classification.