Hejaz
Historically, the Hejaz has always seen itself as separate from the rest of Saudi Arabia. The Hejaz is the most populated region in Saudi Arabia; 35% of all Saudis live there. Hejazi Arabic is the most widely spoken dialect in the region. Saudi Hejazis are of ethnically diverse origins.
Arabic
Charles Ferguson's koine theory (Ferguson 1959) claims that the modern Arabic dialects collectively descend from a single military koine that sprang up during the Islamic conquests; this view has been challenged in recent times. Ahmad al-Jallad proposes that there were at least two considerably distinct types of Arabic on the eve of the conquests: Northern and Central (Al-Jallad 2009). The modern dialects emerged from a new contact situation produced following the conquests. Instead of the emergence of a single or multiple koines, the dialects contain several sedimentary layers of borrowed and areal features, which they absorbed at different points in their linguistic histories.
According to Veersteegh and Bickerton, colloquial Arabic dialects arose from pidginized Arabic formed from contact between Arabs and conquered peoples. Pidginization and subsequent creolization among Arabs and arabized peoples could explain relative morphological and phonological simplicity of vernacular Arabic compared to Classical and MSA.