In the Arabian peninsula of the pre-Islamic period, oral culture has developed rather than written culture due to a special environment and conditions of the desert. The oral culture had an influence on the formation of the literary, which resulted in the emergence of the poetry with an unique rhythm. Given the social and geographical conditions of the peninsula at that time, each tribe was isolated and used 'lughāt', their own dialects. But with the open of the trade route and not frequent correspondence between the tribes, a lingua franca, a common language between the tribes has developed. The poets of several tribes met in 'Ukāẓ' market to exchange the ideas and recite the poem. In other words, a poetic language has played a role of literary koine as a common language. A poetic language was primarily used by the poets, so it was not easy to understand for the public. Nevertheless, a poetic language has played a very important role of integrating many tribes together. The presence of a poetic language, along with the dialects, has formed a diglossia in the Arabian peninsula and also contributed to the codification of the Qur'ān. A poetic language and the dialects linguistically showed the differences and these differences were reflected in the classical Arabic of the Qur'ān. The dialects were separated into the urban dialects in Mecca and Medina and the Bedouin dialects, and geographically divided into the western Ḥijāzi dialects and the eastern Najdi dialects. The urban dialects during the Islamic conquest have rapidly changed compared to the Bedouin dialects, which have lost its own grammatical rules. On the other hand, the Bedouin dialects relatively maintained a linguistic purity because they were geographically isolated. It seems that the language of the Qur'ān and the pre-Islamic poems were more related to Eastern than to Western Arabic. In some respects, the language of the Ḥijāz differed from the language of the Qur'ān and poetry, and this led to assume that the origin of the classical Arabic,the language of the pre-Islamic poetry, lay in the Central or Eastern part of the peninsula possibly in the Najd. The aim of this study is to study Early Arabic in the pre-Islamic period by investigating the language situation and the linguistic differences between the dialects. In particular, this paper will provide a theoretical basis on the diglossia of the language of the poetry and the dialects. Meanwhile, this paper will illuminate the linguistic phenomena which have happened in the dialects such as the adaptation of the spelling of the hamzah, the glottal stop and the loss of the case endings. In addition, this paper will put a light on the origin of the name 'Arabs' and the Northern and Southern Arabic through the distinction of Arabs according to Arab tradition.