Construct case in the context of "Ad-Din"

Ad-Din (Arabic: الْدِّين ad-dīn [ædˈdiːn], "(of) the religion/faith/creed") is a suffix component of some Arabic names in the construct case, meaning 'the religion/faith/creed', e.g. Saif ad-Din (Arabic: سيف الدّين Sayf ad-Dīn, "Sword of the Faith"). Varieties are also used in non-Arabic names throughout the Muslim world, It is used as a family name-suffix by some royal Muslim families, including the imperial Seljuks, Walashmas, Mughals, and the noble Alvi Hyderabadi families.

The Arabic spelling in its standard transliteration is al-Din. 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). This leads to the variant phonetic transliteration ad-Din. 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). At the same time, the Arabic short vowel u is rendered as short o in Persian, thus od-din.