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Shaddah (Arabic: شَدَّة shaddah "[sign of] emphasis", also called by the verbal noun from the same root, tashdid تشديد tashdīd "emphasis"), is one of the diacritics used with the Arabic alphabet, marking a long consonant (geminate). It is functionally equivalent to writing a consonant twice in the orthographies of languages like Latin, Italian, Swedish, and Ancient Greek, and is thus rendered in Latin script in most schemes of Arabic transliteration, e.g. رُمَّان = rummān 'pomegranates'.
When a shadda is used on a consonant which also takes a fatḥah /a/, it is written above the shaddah, while if it had a kasrah (a dash below the consonant indicating that it takes a short /i/ as its vowel), the kasrah is written between the consonant and the shaddah, under the shaddah, rather than in its normal place.
Consonant length in Arabic is contrastive: دَرَسَ darasa means 'he studied' while دَرَّسَ darrasa means 'he taught'; بَكى صَبِي bakā ṣabī means 'a youth cried' while بَكّى الصَّبِي bakkā ṣ-ṣabī means 'a youth was made to cry'. A consonant may be long because of the form of the noun or verb; e.g., the causative form of the verb requires the 2nd consonant of the root to be long, as in darrasa above, or by assimilation of consonants, for example the l- of the Arabic definite article al- assimilates to all dental consonants, e.g. (الصّبي) (a)ṣ-ṣabī instead of (a)l-ṣabī, or through metathesis, the switching of sounds, for example أَقَلّ aqall 'less, fewer' (instead of *أَقْلَل aqlal), as compared to أَكْبَر akbar 'greater'.
A syllable closed by a long consonant is made a long syllable. This affects both stress and prosody. Stress falls on the first long syllable from the end of the word, hence أَقَلّ aqáll (or, with iʻrāb: aqállu) as opposed to أَكْبَر ákbaru, مَحَبَّة maḥábbah 'love, agape' as opposed to مَعْرِفَة maʻrifah '(experiential) knowledge'. In Arabic verse, when scanning the meter, a syllable closed by a long consonant is counted as long, just like any other syllable closed by a consonant or a syllable ending in a long vowel: أَلا تَمْدَحَنَّ a-lā tamdaḥanna 'Will you not indeed praise...?' is scanned as a-lā tam-da-ḥan-na: short, long, long, short, long, short.
- Versteegh, 1997. The Arabic language. p 56.