Within وَتَوَاصَوْا, the alif after «وَ» forms a madd alif: give it two counts. The wāw in «صَوْ», however, is sākin and follows fatḥah, making it a līn letter, not a madd wāw.
The last alif is orthographic after wāw al-jamāʿah and receives no separate sound.
Practice «tawā» and «ṣaw» independently. Do not assume every written alif and wāw represents a new prolonged vowel.
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Madd and līn within tawāṣaw
Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf
Within وَتَوَاصَوْا, the alif after «وَ» forms a madd alif: give it two counts. The wāw in «صَوْ», however, is sākin and follows fatḥah, making it a līn letter, not a madd wāw.
The last alif is orthographic after wāw al-jamāʿah and receives no separate sound.
Practice «tawā» and «ṣaw» independently. Do not assume every written alif and wāw represents a new prolonged vowel.