•   سُكْكَر ← سُكَّر sugar

•   تُفْفَاحَة ← تُفَّاحَة an apple

•   خَرْرَج ← خَرَّج he made sb/sth go out

•   كَلْلَم ← كَلَّم he spoke to

•   سَيْيِد ← سَيِّد lord, mister / Mr

•   نَزْزِل ← نَزِّل he brought down

•   تَبَخْخُر ← تَبَخُّر evaporation

•   تَفَكْكُر ← تَفَكُّر contemplation

•   سَكَتْتُ ← سَكَتُّ I kept quite

•   فَنْنًا ← فَنًّا an art

•   سِرْرٌ ← سِرٌّ a secret

•   عَدُوْوٌ ← عَدُوٌّ an enemy

(a)   In these words, each consonant that has a shadda ( ـّ ) over is comprised of two identical consonants. You can see these consonants in the decomposed words before the arrow.

(b)   The two consonants are basically assimilated (i.e., blended) into one. For this assimilation to happen, the first consonant must have a sukuun ( ـْ ) over it, while the second consonant must have a short vowel or a tanween, that is ( ـً ), ( ـٍ ) , ( ـٌ ), above/below it.

(c)   The shadda ( ـّ ) gets the short vowel (i.e., ( ـَ ), ( ـِ ), ( ـُ )), or the tanween (i.e., ( ـً ), ( ـٍ ), ( ـٌ )) of the second consonant.

(d)   The shadda ( ـّ ) must be preceded by a consonant that has a short vowel over it, as you can see in the list.

(e)   This is a word-level shadda ( ـّ ). That is, it occurs at the level of words.

•   لَمْ يَخْرُجْ جَـمَال ← لَمْ يَخْرُجْ جَّمَال Jamal didn't go out

•   اُكْتُـبْ بِالْقَلَم ← اُكْتُبْ بِّالْقَلَم write with the pen

•   اِسْمَــعْ عَـلِي ← اِسْمَعْ عَّلِي listen to Ali

•   كُلْ لَوْز ← كُلْ لَّوْز eat almonds

•   لَنْ نَذْهَب ← لَنْ نَّذْهَب we will not go

•   قُلْ لَهُم ← قُلْ لَّهُم tell them

•   مَنْ رَسَبَ؟ ← مَنْ رَّسَبَ who failed

(a)   In these phrases/sentences, the last letter of the first word (in orange) is identical to the first letter of the second word (in green). The first one is saakin (i.e. has a sukuun ( ـْ ) over it), while the second is mutaharik (i.e., has a short vowel above/below it).

(b)   In connected speech, the two sounds/letters become one doubled sound. So, the second letter gets a shadda ( ـّ ), while the first letter is not pronounced. It is still written as an essential letter of the first word, however.

(c)   This is a phrase-level shadda ( ـّ ); it occurs a the level of phrases.

•   التْتَمر ← التَّمْر the dates - fruit

•   الثْثُوم ← الثُّوم the garlic

•   الدْدَار ← الدَّار the house

•   الذْذُعْر ← الذُّعْر the panic

•   الرْرَب ← الرَّب the Lord/God

•   الزْزَهْرَة ← الزَّهْرَة the flower

•   السْسَاعَة ← السَّاعَة the watch

•   الشْسَمْس ← الشَّمْس the sun

•   الصْصِدْق ← الصِّدْق the truth

•   الضْضَحِك ← الضَّحِك the laughter

•   الطْطَبْخ ← الطَّبْخ the cooking

•   الظْظُلْم ← الظُّلْم the injustice

•   اللْلَيْل ← اللَّيْل the nighttime

•   النْنَهَار ← النَّهَار the daytime

(a) The letters ت, ث, د, ذ, ر, ز, س, ش, ص, ض, ط, ظ, ل, and ن are the sun letters (also called solar letters). They are named after the word شَمْس which begin with one of them, that is the ش. The remaining letters of the Arabic alphabet are the moon letters (also called lunar letters).

(b) These letters are grouped together because of a pronunciation phenomenon in Arabic, that is assimilation with the definite article الْ. In particular, the assimilation of the لْ, which always has a sukuun over it, to these letters.

(c) This assimilation happens because these letters (more accurately, consonant sounds) are near to the ل when we produced them in oral cavity. That is, there is a slight pause in moving from the ل to these letters when they are consecutive. The لْ, as a result, is assimilated to the following letter forming an identical letter, hence two consecutive identical letters.

(d)   The first letter has a sukuun ( ـْ ) over it, and the second letter has a short vowel over it (fatha ( ـَ ) , kasra ( ـِ ) , Dhamma ( ـُ )). So, they become one, and a shadda ( ـّ ) is added to indicate the doubling.

(f)    This is a word-level shadda ( ـّ ).

•   قَدْ تَنَام ← قَد تَّنَام you may sleep

•   قَدْ تُقْتَل ← قَد تُّقْتَل you may be killed

•   قَدْ تَكَلَّم ← قَد تَّكَلَّم he has spoken

•   هَلْ رَأَيْت؟ ← هَل رَّأَيْت did you see

•   هَلْ رَجِعَ؟ ← هَل رَّجِعَ did he come back

•   بَلْ رَبِيْع ← بَل رَّبِيْع rather it is Rabe'e

(a)   In these two-word phrases, if we examine the last letter of first word and first letter of the second word, we can see that they are phonetically near each other when produced in the mouth.

(b)   The first one has a sukuun ( ـْ ) over it, and the second has a short vowel. In pronouncing them, they become one. This is indicated by the shadda ( ـّ ) on the second letter.

(c)   In writing, however, they are still written separately, in two separate words.

•   أَنْ يَسْمَعَ ← أَن يَّسْمَع to listen

•   أَنْ يَنْظُر ← أَن يَّنْظُر to look

•   مِنْ رُوْسِيا ← مِن رُّوسِيا from Russia

•   عَنْ رَحْمَة ← عَن رَّحْمَة about Rahma

•   مَنْ لَا ← مَن لَّا who doesn't

•   مَنْ لَكِ ← مَن لَّكِ who will take care of you

•   مَنْ مَعِي ← مَن مَّعِي who is with me

•   عَنْ مَاذَا ← عَن مَّاذا about what

•   مِنْ وَاحِد ← مِن وَّاحِد from one

•   عَنْ وَلَدٍ ← عَن وَّلَدٍ about a boy

(a)   In these phrases, the first word is a preposition (i.e. عَنْ and مِنْ) / particle (i.e. أَنْ) that ends in ن which has a sukuun ( ـْ ) over it (this is called the ن saakina). The second word in these phrases begin with ي, ر, ل, م, or و.

(b)   The ن saakina assimilates to the following letter changing it into a doubled sound. This is indicated by shadda ( ـّ ) on the first letter of the second word (i.e. ي, ر, ل, م, and و). Hence, the ن is written but not pronounced.

(c)   Note that the particle أَنْ is followed by a (present) verb, while the prepositions مِنْ and عَنْ are followed a noun or a prepositional phrase (e.g. لَكِ and مَعِي).

•   رَجُلٌ رَحِيْم ← رَجُل رَّحِيْم a merciful man

•   خَيْرٌ وَفِيْرٌ ← خَيْرُ وَّفِيْرٌ abundant of good

•   شَرَابٍ لّذِيْذ ← شَرابِ لَّذِيْذ delicious drink

•   تَمْرٍ يَابِس ← تَمْرِ يَّابِس dried dates - fruit

•   وَلَدًا وَسِيْمًا ← وَلَدَ وَّسِيْمًا a handsome boy

•   صَوْتٍ مُرْعِب ← صَوْتِ مُّرْعِب a frightening voice

(a)   The first word in these phrases ends in a tanween—that is ( ـً ), ( ـٍ ), and ( ـٌ )—pronounced as an, in, un, respectively. The tanween, therefore, ends in a ن that is saakina.

(b)   The ن saakina of the tanween assimilates to the first letter of the following word resulting in a doubled consonant. The doubling of the sound is indicated by a shadda and the short vowel of letter to which the ن saakina assimilates.

•   عَنْ مَا ← عَمَّا about what

•   مِنْ مَا ← مِمَّا from what

•   أَنْ لا ← أَلَّا in order not to

•   مِنْ مَنْ ← مِمَّن from whom

•   عَنْ مَنْ ← عَمَّن about whom

•   إِنْ لَا ← إِلَّا except

•   إِنْ مَا ← إِمَّا if, either 

(a)   These words are two-word particles. The first particle ends in ن saakina; the second particle begins in م or ل.

(b)   The ن saakina assimilates to the ل or م resulting in doubled ل or م. This doubling is indicated by the shadda and the short vowel of the ل / م.

(c)   Since this kind of assimilation takes place at the level of particles (i.e. very short words), the ن of the first particle is completed left out, and the two particles become one, that is عَمَّا, مِمَّا, أَلَّا … etc.