Calligraphy is about the art of combining characters. This refers to the structural relation between the strokes of each character. There are five main styles of calligraphy: seal script, clerical script, regular script, running script, and cursive script. The formal scripts are standardized and used in most occasions, whereas the informal ones are used in less formal occasions, such as in the writing of personal notes or for appreciation. Traditionally, Chinese calligraphy is written only in black ink, and the most highly regarded ink is Huimo, which refers to ink from the Huizhou region in history, part of which is in the present-day Anhui Province. Ink stick is a type of solid ink used in calligraphy and painting in some Asian countries, such as China, Korea, and Japan. Ink stone is a stone mortar for the grinding and containment of ink when writing calligraphy. Paper, another important implement of Chinese calligraphy, is one of the four major inventions of China.