It is spoken throughout East Africa, that is to say Tanzania, (i.e. Tanganyika and Zanzibar), Kenya, Uganda, and a little in Eastern Zaire and Burundi. The purest Swahili is to be found on the island of Zanzibar and along the mainland coastline closest to it. But as you get farther away, both north and south as well as inland, the standard of the language lowers. Therefore, in northern Kenya, the extreme west of Tanzania and its southern borders, the language may be little known or extremely ungrammatical. In Uganda, the language is fairly commonly used except in the south-west. In eastern Zaire and Burundi, the Swahili language has a strong French flavour which is perfectly understandable to someone who knows both languages.
During its first millennium Swahili was enriched with thousands of Arabic words, which were fitted into the Bantu system of noun classes and verb categories. The result was a totally new linguistic structure, that evolved along its own lines.
This offshoot of the Bantu stock was already the medium of an African culture and the treasure trove of many songs, proverbs and stories, when, during the first millenium, the religion of Islam spread peacefully along the coast. Islam is more than dogma and morality, more than ritual and ceremony: Islam is a way of life, a law for the individual as well as for the community, a cosmology that has made the Swahili language into a uniquely beautiful vehicle for the expression of its ideas in literary form.
Prose was until recently practically restricted to utilitarian purposes, but the traditional art of verbal expression in poetry has produced an overwhelming number of valuable works. The traditional poetry can be divided into different groups according to its form and content: it can be epic, lyrical or didactic, as well as religious or secular.
After the First World War writers composed stories and novels in Swahili which added a new dimension to Swahili literature which hitherto had not known fiction as a written form. Prose, formerly largely confined to historic subjects, theology and other such subjects, was successfully used by Shaaban Robert for essay writing. A second new development during the same period was the appearance of poems in the weekly press, mainly in the shairi metre, which had previously belonged chiefly to the oral tradition of secular verse.
When the learned Arab traveller Al-Mas'udi landed at Mombasa in the tenth century, he noted a few words in the local language which he called Zinji or Zanji (owing to the fact that Arabic script does not usually distinguish the vowels, we are not certain of the pronunciation). On the present linguistic evidence, therefore, it is possible to concludethat some form of Proto-Swahili was being spoken along what is now the Kenyan coast before the tenth century.
Swahili was originally spoken as far north as Mogadishu where some place names (such as Shangani, on the sand) are reminders of the former population who were expelled by the Somalis in the sixteenth century and later. In Barawa and Kisima-iu (the latter place name means upper well in Swahili) Swahili is still spoken, as also on the islands along the southern Somali coast.
The southern tip of the Swahili language area lies in northern Mozambique; the town of that name was once a Swahili town, and its original name Msumbiji, is Swahili.
In between Mogadishu and Mozambique lies what has been known since the Middle Ages as the Swahili coast. Arab scholars of that period called it the biladu 's-sawahili (the towns of the coastal people), hence the name Swahili. All the islands off the coast between these places are Swahili speaking, including the Comoro Islands.
Swahili was carried inland by the caravans which bartered Indian cotton cloth for ivory and other wares.