In natural speech, it is always difficult to utter a sound twice in a row, as in عَلْلَم which becomes عَلَّم. It is also relatively difficult to produce two sounds from the same area of articulation consecutively, as in مِنْ رَأْسِكَ which is pronounced as مِرَّأْسِكَ. Therefore, we recourse to assimilation. In Arabic, assimilation الإِدْغَام happens when two identical letters (or rather sounds) or comparatively similar sounds become one geminate (doubled) sound / letter حَرْف مُشَدَّد. This post gives a fairly detailed explanation of assimilation in Arabic.

Why It Happens:

Assimilation in Arabic occurs because it is not easy for a speaker to move an organ of articulation (i.e. the tongue, the lips, the teeth) twice in a row while uttering two identical sounds or sounds produced from the same area of articulation. Without assimilating them, speech will be more like tongue-twisters. Therefore, identical consecutive sounds/letters become single geminates (doubled) sounds. Likewise, sounds/letters that are not identical but share many features become a single geminate sound.

Role Diacritics in Assimilation:

Diacritical marks, namely الفَتْحَة fatHah, الضَّمَّة DHammah, الكَسْرَة kasrah (let’s call them short vowels or Harakaat and the singular is Harakah) and السُّكُون sukuun, play a major role in the occurrence of assimilation. Since the assimilation occurs between two letters / sounds. There are three ways to add diacritics to them: 1) sukuun + Harakah, 2) Harakah + sukuun, and 3) Harakat + Harakah, as in سُكْكَر (سُكَّر), رَدَدْتُ, and رَدَدَ, respectively. The type of Harakah on the letters determines whether assimilation is mandatory, optional, or impossible.

Types of Assimilation:

Based on the kind of diacritical marks on the identical / similar letters, there are three types of assimilation in Arabic: mandatory, optional, and impossible.

Mandatory Assimilation:

This type of assimilation happens when the first of the two identical letters has a sukuun, while the second has a harakah (i.e. fatHah, kasrah, or DHammah). This happens in single words or two successive words, as in these examples:

Not assimilated	Assimilated
نَزْزَل	نَزَّلَ  ‘move down’
سَلْلَمَ	سَلَّمَ ‘hand in’
حَبْبَ	حَبَّ ‘to love’
سَكَتْتُ	سَكَتُّ ‘I kept quiet’
نُظْظِفَ	نُظِّفَ ‘was cleaned’
صَدِيْقَكْ كَرِيْم ‘your friend is generous’	صَدِيْقَكَّرِيْم
كُلْ لَحْم ‘eat meat!’	كُلَّحْم
الخَيْل لِي ‘the horse is mine’	الخَيْلِّي
عَنْ مَا	عَمَّا ‘about what’
لَمْ يَرْجَعْ عَلِي ‘Ali didn’t come back’	لَمْ يَرْجَعَّلِي

However, if the last letter of the first word is yaa’ or waaw (both with sukuun) followed by another word that begin the same letters this type of assimilation does not occur, as in these examples:

(أ) يَجْرِيْ يَحْيَى ‘Yahya runs’

(ب) يَمْشِيْ يَاسِر ‘Yasir walks’

(ج) يَصْحُو وَسِيْم ‘Waseem wakes up’

In (أ) and (ب), the yaa’ of the first word does not assimilate with that of the second word, hence it is not pronounced as يَجْرِيَّحْيَى or يَمْشِيَّاسِر. In (ج), the two waaws are not pronounced as يَصْحُوَّسِيْم. In other words, assimilation does not take place.

Optional and/or Impossible Assimilation:

This assimilation is optional because the two identical letters have two successive Harakaat (i.e. no sukuun); therefore, its occurrence is governed by certain rules and conditions, as detailed below:

(1) If the two identical letters are the same word, the must be assimilated, as in these examples:

مَرَرَ = مَرَّ ؛ شَدَدَ = شَدَّ ؛ مَلِلَ = مَلَّ

If the two identical letters are in two successive words, assimilating them is optional, as in these examples:

جَعَلَ لَك = جَعَلَّكَ ؛ خَرَجَ جَمِيْل = خَرَجَّمِيل ؛ إِنَ البَابَ بَعِيْد = إِنَّ البَابَّعِيْد

If the two identical letters are in two successive words, and the letter that precedes the first identical letter has a sukuun, they must not be assimilated, as in these examples:

بَيْتُ تَوْفِيق ؛ شَهْرُ رَمَضَان ؛ عَيْنُ نَادِيَة

(2) If the two identical letters are at the beginning of the word, they must not be assimilated, as in these examples:

تَتَذَكَّر ؛ تَتَعَلَّم ؛ تَتَظَاهَر

(3) If the first letter of the two identical letters is already assimilated with a preceding letter, assimilation must not occur because it cannot happen between three letters, as in theses examples:

قَرَّرَ ؛ بَرَّرَ ؛ قَزَّزَ ؛ حَقَّقَ

(4) If the two identical letters are part of tri-consonantal nouns (i.e. nouns composed of three consonants) that are weighed on the Morphological Measure as فَعَل, فُعُل, فِعَل, or فُعَل (i.e. the ‘ain of these nouns has a harakah), they must not be assimilated, as in these examples:

شَلَل ، سَبَب ؛ سُرُر ، ذُلُل  ؛ لِمَم ، كِلَل  ؛ أُمَم ؛ جُرَر

(5) If the Harakah on the second identical letter is not original (i.e. caused by some other speech phenomena, as explained below), they must not be assimilated, as in these examples:

أَحْبِبِ الخَيْرَ ؛ اُكْفُفِ الشَّر

In the first sentence, the Harakah that accompanies the second ب is originally a sukuun because this verb is in the imperative mood, hence it must be مَجْزُوم jussive, i.e. assigned a sukuun. However, the sukuun changes to kasrah due to the sukuun on the alif of the following word (الخَيْر). This is called اِلْتِقَاء السَّا كِنَيْن juxtaposition of two sukuuns. This phenomenon requires that the first sukuun be replaced with either fatHah or kasrah.

(6) If the two identical letters are ت + ت and occur in the middle of the word, they in most cases must not be assimilated, as in these examples:

اِقْتَتَلَ ، اِسْتَتَرَ ، اِبْتَتَر

Two-word Assimilation:

This kind of assimilation is obligatory in Quranic Arabic (i.e. while reading the Quran). It is also common in everyday spontaneous speech. It occurs between two words, namely when the last letter of the first word share many features with the first letter of the second word, that is they are produced from the same place of articulation. For this reason, they assimilate and become one geminate letter. The most common assimilations of this type are:

(1) The assimilation of لْ (laam with sukuun) of the first word to the ر (raa’ with Harakah) of subsequent word. The resulting sound is a doubled رّ, as in these examples:

قُلْ رَبِّي  = قُرَّبِّي  ؛ لَمْ يَقُلْ رَحِيْم = لَمْ يَقُرَّحِيْم

(2) The assimilation of the ق (qaaf with a Harakah) of the first word to the ك (kaaf with a Harakah) of the following word. The resulting sound is a geminate كّـ, as in these examples:

خَنَقَ كَامِل = خَنَكَّامِل ؛ خَلَقَ كُلَّ النَّاس = خَلَكُّلَ النَّاس / خَلَكُّلَّنَّاس

(3) The assimilation of لْ in the الـ (the definite article in Arabic) to the subsequent sun letter. The resulting sound is a geminate sun letter. The الـ is considered a separate word in Arabic grammar. Here are examples:

الشَمْس = اَشَّمْس ؛ النُور = اَنُّور

(4) The assimilation of the نْ (nuun with sukuun, called ­in Arabic النُّون السَّاكِنَة) to ي, ر, م, ل, و, or ن of the subsequent word. The resulting sound is ـيّـ, رّ, مّ, لّ, وّ, or ـنّـ, as in these examples:

مَنْ لَم = مَلَّم ؛ مَنْ يَرْمِي = مَيَّرْمِي ؛ مَنْ رَأى = مَرَّأى ؛ مَنْ وَضَعَ = مَوَّضَعَ ؛ مَنْ نَعْرِف = مَنَّعْرِف

(5) The change of ن to م when it is followed by ب, as in these examples:

مِنْ بَعْد = مِمْبَعْد ؛ أَنْبِئْهُم = أَمْبِئْهُم