Arabic definite article
The Arabic spelling in its standard transliteration is "ad-Din," due to the phonological rules involving "sun letter" (حرف الشّمسيّة "hurfu ’sh-Shamsiyyah"), the Arabic letter د ("dāl") is 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 is in the nominative case, assimilates the following "a"-, thus manifesting into "ud-Din" in Classical and Modern Standard Arabic.