The harmony of Yin and Yang is the basis of the Chinese concept of the universe. Simply put, these are any two opposites combined - feminine and masculine, dark and bright, loud and quiet, sour and sweet, high and low.
In the West, this concept is often understood quite simplistically - as negative and positive. In fact, it's not quite so. In China, these two opposites aren't good or bad; they merely need to be in harmony. Their unity is symbolized by a circle in which a small point of Yin is in the Yang's half, and vice versa.
The harmony of Yin and Yang is one of the feng shui postulates. A certain balance should also be observed in each room of any home. A room that does not comply with this balance will be at least uncomfortable to live in.
There are certain rules for designing a balanced and thought-out interior. For example, a bedroom and office are Yin. It would be inappropriate to use excessively bright colors in these rooms, as well as angular furniture. A living room on the contrary should be bright.
However, it's best to avoid the use of strictly Yin or purely Yang elements in any room. Listen to your instincts and try to make your interior maximally comfortable and harmonious.
If you need to add Yang you can intensify the lighting, add bright colors to the interior, use high furniture, and maybe even hang a clock with a pendulum. If you need to increase Yin, you can use less powerful light bulbs, add dark colors to the interior, as well as use low furniture and dark curtains.
The Yin and Yang also shows in geographic locations, which may either be unfavorable, or even unsuitable for human habitation. For instance, both, the hot deserts (Yang) and cold Antarctic (Yin) aren't too suitable for living - such extremes can be tolerated for only a short time. It's important to follow the harmonious combination of the two everywhere, such as when choosing the site for a house or an office. If there are some issues in that geographic location, some of them may be fixed while others cannot.
Let's talk more about these two elements in nature. According to the classic landscape school of feng shui all the land is divided into two types: rock and water. The rocks are the bones of the land, while water is its blood.
Water is dynamic, and belongs to Yang; mountains are static, and belong to Yin. Harmony comes when these two are in equilibrium. If there are mountains, but no water - the energy flow will diminish and fade.
If there's a lot of water, but no mountains - the energy becomes hard to tame and unmanageable. For any place to possess the power, the energy should not stagnate, but it should not dissipate either. So, they both should be represented in the landscape to make for a good geographical location for a home.
As you can see, there are quite a number of aspects that one may want to learn about the Yin and Yang, in order to be able to balance them the best way.