The concept of "unity in duality" as underlying the nature of the Cosmos is fundamental in the philosophy of Heraclitus, one of the Presocratics. Note that the Heracletian duality has nothing to do with an ontological dualism.
Indian philosophy has a distinct dualistic strand, known as Samkhya. In this theory, Yin corresponds to Prakrti and Yang to Purusha.
Gnosticism and Zoroastrianism posit a supernatural dualism to explain suffering in this world.
Indonesia has the motto: "Bhinneka Tunggal Ika" that is "Unity in diversity" which originates from a quotation of an Old Javanese poem. This idea is similar to yin and yang philosophy. "Tunggal" means the only one, as in "anak tunggal" (only child).

• Yin-yang are Opposing
o Yin and yang describe the polar effects of phenomena. In viewing any one phenomenon (or the comparison of two phenomena), yin and yang describe the opposing qualities inherent in it. For instance, winter and summer would be the yin and yang, respectively, of the year.
• Yin-yang are Mutually Rooted
o Yin and yang are two complementary qualities. That is to say, the yin and the yang aspect of any one phenomenon will, when put together, form the entire phenomenon. Yin-yang is a philosophy of duality. This is the reason the Chinese word has no "and" between yin and yang - the term always expresses the two making up the one. In the example above, winter plus summer makes up the whole year.
• Yin-yang Mutually Transform
o The maximum effect of one quality will be followed by the transition toward the opposing quality. In other words, once the maximum Yang aspect has manifest, such as the long days of summer, this will be followed by the transition toward the Yin aspect, with the shortening of the days as winter approaches.
• Yin-yang Mutually Wax and Wane
o The Yin and yang aspects are in dynamic equilibrium. As one aspect declines, the other increases to an equal degree. For instance, in the cycle of the year, the long days of summer gradually shorten and the nights gradually lengthen as winter approaches. Throughout the process, however, the length of each day is constant (the equilibrium) while it is only the relative length of light and darkness that changes (is dynamic).
Yin and yang are neither substances nor forces. They are the terms used in a system of dualistic qualification which can be applied universally. Further dividing Yang and Yin into their respective Yin and Yang aspects, yields four combinations: the Yin of the Yang, the Yang of the Yang, the Yin of the Yin, and the Yang of the Yin. This allows an endless scale of universally defined qualities, which is foundational to classical Chinese thought, as seen in the Tao Te Ching, and science, as seen in the Yellow Emperor's Huangdi Neijing.

In Chinese philosophy, yin and yang are generalized descriptions of the antitheses or mutual correlations in human perceptions of phenomena in the natural world, combining to create a unity of opposites in the theory of the Taiji. The term liang yi, also known as Yin and Yang "Although the Chinese lay claim to the symbol, the earliest yin-yang symbol was found inscribed in stone in Korea." Thus, many historians believe that the concept of Yin and Yang is actually Korean.
The concept of yin and yang describes two opposing and, at the same time, complementary (completing) aspects of any one phenomenon (object or process) or comparison of any two phenomena. They are universal standards of quality at the basis of the systems of correspondence seen in most branches of classical Chinese science and philosophy, traditional Chinese medicine being an example.
Two qualities

Yin qualities are characterized as soft, slow, substantial, wet, cold, conserving, tranquil, gentle, and corresponding to the night.

Yang qualities are characterized as hard, fast, insubstantial, dry, hot, excited, aggressive, and corresponding to the day.