Although there is an undeniable link between Arabic calligraphy and the Islamic religion, the Arabic script existed before Islam. In the earliest examples of Arabic writing, there are two distinct styles; one is more fluid and rounded in shape, and the other is more geometric and sharp. These came to be known as naskh and kufic repesctively, and this broad distinction is maintained to this day. However, as diverse populations throughout the Middle East began using the script, many variations on each style developed, and Arabic writing now displays a great degree of variation.
It is difficult to draw a line between Arabic calligraphy as a vehicle for communication, and Arabic calligraphy as an art form. A common handwriting style in everyday use by most literate speakers of Arabic is Ruq'ah or Riq'a. It is generally believed to be the easiest style to write and the clearest style to read.
Calligraphic writing has traditionally been held in high regard as an art form in the Islamic world. Islam discourages the depiction of the human form, so Islamic art has tended to combine geometric shapes with depictions of plants and animals, and calligraphic writing. Examples of this can be found inside almost every mosque. Some popular styles of Arabic calligraphy are thuluth, an elegant style with elongated ascenders and sloping descenders, nasta'liq, having short vertical strokes and elongated horizontal strokes, generally considered the core script of the Persian writing tradition, diwani, a decorative style historically used for secret documents in the Ottoman courts, characterized by its intertwining letters which make it difficult to read, write or forge.
In Arabic calligraphy, the rules governing letter placement are somewhat less rigid than in everyday writing. As long as the placement does not hamper comprehension of the word(s), letters can be placed above, below or within each other or joined in unconventional ways. Some calligraphers arrange and proportion the letters to satisfy mathematical criteria. The spaces between letters may be strewn with decorative marks having no orthographic value. In a calligram, an inscription takes the form of a flower, bird or other animal, or, less frequently, an inanimate object such as a ship.