They will tell you that you have combination skin when you have oily areas as well as dry areas on your face. The oily areas are usually in the forehead, nose and chin, or the T zone. This happens because the T zone is where there is a higher amount of oil glands or sebaceous glands.
These glands secrete an oily, waxy, protective substance called ‘sebum’. Sebum waterproofs and lubricates your skin, it keeps your skin texture smooth and prevents it from aging. It also works with your heat regulatory glands and plays an important role in different climatic conditions. When it is hot, sebum emulsifies with sweat to create a protective layer that delays dehydration of the skin. In colder climates, sebum becomes more lipid or oily. In this case when it coats the skin it effectively repels rain.
Conclusively the job of these sebaceous glands is to protect your skin with a coating of sebum. If your skin is dry these glands will produce excess sebum in an effort to protect your skin. And you think you have combination skin. If you moisturise your ‘combination’ skin with nutritious, fine molecular oils the dry areas will get moisturized and your sebaceous glands will not have to over protect your skin. They will produce less sebum and you will not have oily and dry patches. Instead your skin will be even toned.
Please do not treat your dry patches and oily patches separately as there is no such thing as ‘combination’ skin.