Syntactic gemination, or syntactic doubling, is an external sandhi phenomenon in Italian and some other Western Romance languages. It consists in the lengthening (gemination) of the initial consonant after words of certain categories.
The phenomenon is variously referred to in the English language as word-initial gemination, phonosyntactic consonantal gemination, as well as under the native Italian terms: raddoppiamento sintattico (RS), raddoppiamento fonosintattico (RF), raddoppiamento iniziale, rafforzamento iniziale (della consonante)
In standard Italian, syntactic doubling occurs after the following words (with exceptions described below):
- all stressed ("strong") monosyllables (monosillabi forti) and many unstressed ("weak") monosyllables: è, e, o, a, da, fra, che, se, ma, più, può, gru, gnu, re, blu, tre, ciò, sì, già, giù, là, lì, qua, qui, né, sci, tè
- Example: Andiamo a casa [anˈdjaːmo akˈkaːsa], Let's go home
- all polysyllables stressed on the final vowel (this and the previous types are called oxytone words)
- Example: Parigi è una città bellissima [paˈriːd͡ʒi ɛ una t͡ʃitˈtabbelˈlissima], Paris is a very beautiful city
- some paroxytone words (stress on the second last syllable): come, dove, qualche, sopra (sovra)
- Example: Come va? [ˈkomevˈva], How are you?
The cases of doubling are commonly classified into "stress-induced doubling" and "lexical".
"Syntactic" means that gemination spans word boundaries, as opposed to the ordinary geminated consonants as e.g., in "grappa". The emergence of syntactic doubling has traditionally been explained by a diachronic phenomenon of the loss of terminal consonants in Italian during its evolution from the Latin language (e.g. ad->a, quid->qui, etc.), however more recent research also pays attention to synchronic aspects.
Syntactic doubling is standard native pronunciation in Tuscan, Central (both "stress-induced" and "lexical") and Southern Italy (only "lexical"), including Sicily and Corsica. In Northern Italian versions of standard Italian, including Sardinia, speakers have difficulty using it correctly, because this feature is neither present in their dialectal substratum nor shown by the spelling.
Syntactic doubling usually is not reflected in spelling, unless a new word is produced by the fusion of the two: "chi sa"-> chissà ("who knows" in the sense goodness knows). In phonetic transcription, e.g., in the Zingarelli dictionary, the words that lead to syntactic doubling have an asterisk appended; e.g., the preposition "a" is transcribed as /a*/.
Syntactic doubling is not part of the normal grammar programmes in Italian schools, so that people who dodn't have a specific training in phonetics (such as people who studied linguistics or attended courses in theatrical pronunciation) typically don't recognize it as a standard feature of the language. Because of this, many Italian speakers are persuaded that syntactic doubling is a pronunciation error typical of the centre and south of Italy, with the consequence that northern speakers don't even try to acquire this feature, and central-southern speakers try to avoid it when speaking in formal situations.
Exceptions to the basic rules 
Consonant gemination does not happen when:
- a pause occurs on the boundary of word in question
- In particular, the initial gemination may be conditioned by syntax, determining the likelihood of pause. For example, in the phrase "La volpe ne aveva mangiato metà prima di addormentarsi" ("The fox had eaten half of it before falling asleep"), there is no gemination after metà if there is even a slight pause, as prima is part of the adjunct, a sentence element easily isolated phonologically from the main clause within the prosodic hierarchy of the phrase.
- the stressed final vowel is lengthened 
- a sharp break or change in the pitch on the word boundary happens
There are other considerations, especially in various dialects, so that the initial gemination is in fact subject to complicated lexical, syntactic, and phonological/prosodic conditions.
- Migliorini, Bruno; Tagliavini, Carlo; Fiorelli, Piero; Bórri, Tommaso Francesco (31 January 2008). "come". Dizionario d'ortografia e di pronunzia (DOP) (in Italian). Rai Eri. Retrieved 26 February 2010.
- Doris Borrelli (2002) "Raddoppiamento Sintattico in Italian: A Synchronic and Diachronic Cross-Dialectical Study" (Outstanding Dissertations in Linguistics) Routledge, ISBN 0-415-94207-1
- Absalom, Matthew, Stevens, Mary, and Hajek, John, "A Typology of Spreading, Insertion and Deletion or What You Weren’t Told About Raddoppiamento Sintattico in Italian", in "Proc. 2002 Conference of the Australian Linguistic Society", Macquarie University (e-print pdf file)
- Nespor, Marina & Irene Vogel (1986). Prosodic Phonology. Dordrecht: Foris.
- Syntactic Doubling
- Robert A. Hall, Jr. "Initial Consonants and Syntactic Doubling in West Romance", Language, Vol. 40, No. 4 (1964), pp. 551–556
- Loporcaro M, "L’origine del raddoppiamento fonosintattico: saggio di fonologia diacronica romanza", Francke Verlag, Basel, 1997.
- Absalom, Matthew, and Hajek, John (2006), "Raddoppiamento sintattico and Prosodic Phonology: A Re-evaluation", In Allan, Keith, Eds. Proceedings 2005 Conference of the Australian Linguistics Society, Monash University.