Due to the phonological rules involving the "sun letter" (حرف الشّمسيّة hurfu ’sh-Shamsiyyah), the Arabic letter د (dāl) is an assimilated letter of the Arabic definite article ال (al).
The first noun of the compound must have the ending -u, which, according to the assimilation rules in Arabic (names in general are in the nominative case), assimilates the following a-, thus manifesting into ud-Din in Classical and Modern Standard Arabic.
However, all modern Arabic vernaculars lack the noun endings.
Thus, the vowel of the definite article in them is pronounced in full as either a or e (the latter mostly in Maghreb and Egypt).
In practice, romanizations of Arabic names containing this element may vary greatly, including: Examples of names including this element are: In modern times in English-speaking environments, the name Uddin has sometimes been used as if it was a separate surname.