The Rules of Nun Sakinah and Tanwin

Definitions

Nun Sakinah (النُّون السَّاكِنَة) is a Nun without a vowel that remains in both joining and stopping; it occurs in nouns, verbs and particles, in the middle or at the end of a word. Tanwin (التَّنْوِين) is an extra silent Nun added to the end of a noun in pronunciation but not in writing; it is pronounced when joining and dropped when stopping, and it is written in the Mushaf as a doubled vowel: ﴿عَذَابٌ عَظِيمٞعَذَابٌ عَظِيْمٌ﴾ (Al-Baqarah 2:7).

When they meet the letters of the alphabet they have four rules: Izhar (الإِظْهَار), Idgham (الإِدْغَام), Iqlab (الإِقْلَاب) and Ikhfa' (الإِخْفَاء); an Alif never comes after them.