There are three major groups of dialects in Saudi Arabia - Hijazi, spoken on the western coast, in Jidda, Taif, and the holy cities of Mecca and Medina; Najdi, spoken in and around Riyadh, in the north central part of the country; and Shargi, spoken in the oil-rich eastern reqion. While the Najdi dialect enjoys prestige by virtue of its conservatism and relative closeness to Classical Arabic and the fact that it is the dialect of the royal family, the Hijazi dialect is used throughout the country for government and commercial purposes, and has become the most widely-understood dialect in the Arabian Peninsula. The Hijazi dialect is not "pure" Saudi Arabic, and reflects recent borrowings from other dialects, especially Egyptian, Jordanian and Palestinian; for this reason, sometimes one word or expression was selected from several which may be heard, and sometimes alternative expressions are introduced, since two or even three forms may be in frequent use.
Since there is no "standard" Hijazi dialect, we will consider the dialect as spoken in Jidda. Preference will be given to usage of Jidda, with a few of the most common words from Najdi and from other cities introduced for recognition and identified as such. Preference for modern words and structures, despite the fact that this sometimes means rejecting an older, more Saudi usage would also be given preference. This dialect has been designated "urban" Hijazi to distinguish it from Bedouin dialects also native to the Hijaz region.
The pronunciation of some sounds in Hijazi is variable. There are three interdental consonants (variations of 'th') which may be pronounced as they are in Classical Arabic and in Najdi, as for example in /thalaatha/, or as they are in Egyptian and Palestinian, which would be /talaata/. The latter type of pronunciation is more common in Jidda. (excerpt taken from the FSI Saudi Arabic Language Course)