In الضَّالِّينَ, the article lām assimilates into ḍād. In ضَا لّ, madd alif is followed by a doubled lām in the same word, producing madd lāzim kalimī muthaqqal of six counts.
Near the end, a separate madd yāʾ precedes the final nūn. If you stop, the nūn becomes sākin and you have madd ʿāriḍ li-s-sukūn (2, 4 or 6 counts). The two madd types coexist because their causes differ: a permanent shaddah versus pausal sukūn.
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Aḍ-ḍāllīn: two different madd types
Al-Fātiḥah: Integrated Tajwīd, Madd and Pausing Practice
In الضَّالِّينَ, the article lām assimilates into ḍād. In ضَا لّ, madd alif is followed by a doubled lām in the same word, producing madd lāzim kalimī muthaqqal of six counts.
Near the end, a separate madd yāʾ precedes the final nūn. If you stop, the nūn becomes sākin and you have madd ʿāriḍ li-s-sukūn (2, 4 or 6 counts). The two madd types coexist because their causes differ: a permanent shaddah versus pausal sukūn.