Syntactic gemination, or syntactic doubling, is an external sandhi phenomenon in Italian, some Western Romance languages, and Finnish. It consists in the lengthening (gemination) of the initial consonant in certain contexts.
The phenomenon is variously referred to in English 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)
"Syntactic" means that gemination spans word boundaries, as opposed to word-internal geminate consonants as in [ˈɡatto] "cat" or [ˈanno] "year". 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: a, blu, che, chi, dà, do, e, è, fa, fra, fu, già, ha, ho, là, ma, me, né, o, può, qua, qui, re, sa, sé, so, sto, su, tra, tre, tu, va
- Example: Andiamo a casa [anˈdjaːmo a‿kˈkaːza], 'Let's go home'
- all polysyllables stressed on the final vowel (that and the previous types are called oxytone words)
- Example: Parigi è una città bellissima [paˈriːdʒi ɛ una tʃitˈta‿bbelˈlissima], 'Paris is a very beautiful city'
- some paroxytone words (those whose stress is on the second-last syllable): come, dove, qualche, sopra (sovra)
- Example: Come va? [ˈkome‿vˈva], 'How are you?'
Articles, clitic pronouns (mi, ti, lo, etc.) and various particles do not cause doubling in Standard Italian. Phonetic results such as occasional /il kane/ → [ikˈkaːne] 'the dog' in colloquial speech are transparent cases of synchronic assimilation.
The cases of doubling are commonly classified as "stress-induced doubling" and "lexical".
Lexical syntactic doubling has been explained as a diachronic development, initiating as straightforward synchronic assimilation of word-final consonants to the initial consonant of the following word, subsequently reinterpreted as gemination prompts after terminal consonants were lost in the evolution from Latin to Italian (ad > a, et > e, etc.). Thus [kk] resulting from assimilation of /-d#k-/ in Latin ad casam in casual speech persists today as a casa with [kk], with no present-day clue of its origin or of why a casa has the geminate but la casa does not (illa, the source of la, had no final consonant to produce assimilation).
Stress-induced word-initial gemination conforms to phonetic structure of Italian syllables: stressed vowels in Italian are phonetically long in open syllables, short in syllables closed by a consonant; final stressed vowels are by nature short in Italian, thus attract lengthening of a following consonant to close the syllable. In città di mare 'seaside city', the short final vowel of città thus produces [tʃitˈtaddiˈmaːre].
Syntactic gemination is the normal native pronunciation in Tuscany, Central Italy (both stress-induced and lexical) and Southern Italy (only lexical), including Sicily and Corsica. In Northern Italy speakers use it inconsistently because the feature is not present in the dialectal substratum and is not usually shown in the written language unless a single word is produced by the fusion of two constituent words: "chi sa"-> chissà ('who knows' in the sense of 'goodness knows').
It is not formally taught in normal grammar programmes in Italian schools so most speakers do not recognise it as a standard feature of the language; indeed, northern Italian speakers tend to mistakenly consider it to be a regional pronunciation typical of Central and Southern Italy. Thus, northern speakers often do not try to acquire the feature.
It does not occur in the following cases:
- a pause is at the boundary of words in question;
- In particular, initial gemination may be conditioned by syntax, which determines 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 that is 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 occurs in the pitch on the word boundary.
There are other considerations, especially in various dialects, so that initial gemination is subject to complicated lexical, syntactic and phonological/prosodic conditions.
In Finnish, the phenomenon is called rajageminaatio or rajakahdennus, alku- or loppukahdennus (boundary gemination, boundary lengthening). 
It is triggered by certain morphemes. If the morpheme boundary is followed by a consonant, then it is doubled, if by a vowel then a long glottal stop is introduced. For example, "mene pois" is pronounced "meneppois" [menepːois] and "mene ulos" [meneʔ:ulos].  Following Fred Karlsson (who called the phenomenon "initial doubling"), these triggering morphemes are called x-morphemes and marked with the superscripted 'x', e.g., "sadex". 
Maltese does not itself feature syntactic gemination, but it predominantly borrows Sicilian and Italian verbs with a geminated initial consonant, e.g. (i)kkomprenda, (i)pperfezzjona from Italian comprendere, perfezionare. Though reinforced by native verbal morphology (and hence also restricted to verbs), this phenomenon likely goes back originally to syntactic gemination in the source languages.
- Doris Borrelli (2002) "Raddoppiamento Sintattico in Italian: A Synchronic and Diachronic Cross-Dialectical Study" (Outstanding Dissertations in Linguistics) Routledge, ISBN 0-415-94207-1
- "Accademia della Crusca – Archive.today". Retrieved 26 February 2010.
- 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.
- Suomi, Kari & Toivanen, Juhani & Ylitalo, Riikka (2008). Finnish sound structure – Phonetics, phonology, phonotactics and prosody (PDF). Oulu University Press. ISBN 978-951-42-8984-2.CS1 maint: multiple names: authors list (link)
- Karlsson, Fred: Suomen kielen äänne- ja muotorakenne. Porvoo: WSOY, 1982. ISBN 951-0-11633-5.
- Syntactic Doubling
- Robert A. Hall, Jr. "Initial Consonants and Syntactic Doubling in West Romance", Language, Vol. 40, No. 4 (1964), pp. 551–556.
- Loporcaro Michele, "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" (also PDF), In Allan, Keith, Eds. Proceedings 2005 Conference of the Australian Linguistics Society, Monash University.