Background
Its important to appreciate the significance of language in the Arabic culture to understand the development of Arabic calligraphy. Arabs consider language an art form which is evident in their deep love and respect for poetry. For this reason, the miracle that was given to our prophet Mohammad (SAW) is the Qur'an. Even the best of the best poets in the Arabian Peninsula could not compete with the grandeur of the language and rhythm of the Qur'an, thus admitting that this book is not the creation of any man, but a god.
When the prophet died, the Qur'an was passed on from lip to lip by people who have memorized the whole Qur'an. Many of those people were killed in battles, which alarmed the Muslims. As a result, to preserve this miracle the Sahaba requested from Abu Bakr Al-Sideeq (the Caliph) to compile the Qur'an to prevent it from being lost. The Qur'an as we know it today is credited to a later Caliph, Uthman ibn Affan, who verified the text and published the Qur'an in writing 18 years after the prophet's death.
Furthermore, it is worth noting that the first verses that were delivered to the prophet (SAW) were the following:
Read! In the Name of your Lord, Who has created,
Has created man from a clot.
Read! And your Lord is the Most Generous,
Who has taught (the writing) by the pen
Has taught man that which he knew not.
-Surah Al-Alaq (The Clot, 96: 1-5)
This is the starting point for the development of the Arabic calligraphy. Arabs felt the need to master the art in which the words of God would be seen and read. And so the journey of developing the script system that we know it today began, based on the integration of "artistry and scholarship", representing the power and beauty of abstract lines in which energy flows forming the letters and words.
As the Islamic empire expanded towards the non-Arab speaking world, the Arabic calligraphy became an "abstract expression of Islam". This lead to two important factors that changed Arabic calligraphy, the first is the need to reform the written script into a clear and universal manuscript in order for non-Arab Muslims to learn the Arabic language. This is done gradually by the introduction of dots, vowel signs, and other marks known as "Tashkeel", usually written with different colors to be distinguished. The second factor is the introduction of new regional calligraphy. This is mainly influenced by the integration of Arabic calligraphy into non-Arabic cultures forming new calligraphic styles, most notably those that were formed in Persia and Turkey.
9th-10th century Quran in Early Kufic style, Iraq
11th century Quran in Eastern Kufic style, Iran
Styles
Kufi: This is perhaps the oldest calligraphic style which was developed in Kufa in Iraq, hence the name. A number of variations have been introduced to this style. It is characterized by its pronounced angularity, geometrical construction and being considerably wider than it is high. Because of its adaptability to ornamental form, it remains favorable for use in exterior and interior decorations.
Thuluth: This is a cursive script that was formulated during the Umayyad Dynasty. The name means "one third" which is supposed to be the proportion between straight lines and curves. It is characterized by its curved letters with barbed heads, elaborate graphics and it is popular for being used in inscriptions, titles and headings.
Naskh: This cursive script is considered the most popular calligraphic style used. Derived from Thuluth, it was probably redesigned during the Abbasid Dynasty. It is relatively easy to read and write; it uses short horizontal stems, equal vertical depth above and below the medial line, and generally well spaced.
Riq'a: Derived from Thuluth and Naskh, it is a simplified modern cursive style and considered the easiest script for daily handwriting. Letters are similar to those of Thuluth but are smaller, rounded and densely structured with short horizontal stems, and barbed heads are never used.
Ta'liq: Also known as Farsi, it is a cursive script developed by the Persians. This style is considered the native calligraphy among the Persian, Indian, and Turkish Muslims. A lighter and more elegant version of Ta'liq, called Nasta'liq, has been developed by combining it with Naskh. Ta'liq and Nasta'liq are both used extensively in Persian literary works but not for the Qur'an.
Deewani: This style is developed from Ta'liq by the Turks during the Ottoman Empire. It is excessively cursive and its letters are unconventionally joined together. Deewani also developed an ornamental variety called Deewani Jali. The spaces between the letters are filled with decorative devices which do not necessarily have any linguistic value.
Applications
As pictorial ornaments were forbidden in Islam to prevent Idolatry, geometrical patterns and calligraphy became the main decoration elements. For this reason, artists have applied those two elements outside the world of elegant books and into architecture and many crafts such as rugs, woodworking and pottery. But calligraphy have been a decoration element long before Islam, and perhaps the best example for this is the Mu'allaqat (The Hanging Poems), a group of seven long poems written by the best poets of the pre-Islamic period. Those poems were written in gold and hung over the Kaaba as a decoration, showing a very early application of calligraphy as a decorating element.
Today, calligraphy stands as an art on its own, establishing itself as a subject for artists that is open to experimentation. Many artists are specialized in this field and are creating works with innovative approaches to calligraphy in the hopes of developing this art further. More on this will come in the following parts.
The calligraphy decoration of the "kiswah", the cover of the Kaaba
Calligraphy decoration on the entrance of the Taj Mahal
More info can be found here and here and here