thirds

Thirds is the concept of dividing something into three equal parts. It appears in mathematics as the fraction 1/3, in music as an interval, and in visual arts and everyday language as a way of describing proportional relationships. The term is used across disciplines to refer to a one-third portion.

In mathematics, one third is the rational number equal to one divided by three. It is represented as 1/3 and as a recurring decimal 0.333…. Beyond simple fractions, thirds appear in ratios, proportions, and measurements that require equal division.

In geometry, trisecting a segment or an angle means dividing it into three equal parts. Segment trisection is always possible, whereas angle trisection for a general angle cannot be achieved with only a straightedge and compass, a classical result in geometry.

In music, a third is an interval spanning three letter names. A major third covers four semitones, and a minor third covers three semitones. Chords built from stacking thirds, known as triads, are fundamental to Western harmony and tonal music.

In photography and design, the rule of thirds uses two equally spaced vertical lines and two horizontal lines to divide the frame into nine equal parts. Placing important elements along these lines or at their intersections is said to create more balanced and engaging compositions.

In everyday language, thirds often refer to one portion of three equal parts, as in “a third of the cake.” In legal or business contexts, the term may also relate to third parties—external entities distinct from the primary participants.