Shaddah (Arabic: شَدّة 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. رُمّان = rummān 'pomegranates'.
Combination with other diacritics
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.
Significance of marking consonant length
Consonant length in Arabic is contrastive: دَرَسَ darasa means 'he studied' while دَرَّسَ darrasa means 'he taught'; بَكى صَبي bakā ṣabiyy means 'a youth cried' while بَكّى الصَّبي bakkā ṣ-ṣabiyy 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-ṣabiyy instead of (a)l-ṣabiyy, 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 ákbar, مَحَبّة maḥábbah 'love, agape' as opposed to مَعْرِفة 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.