Sandhi (Sanskrit: सन्धि sandhi [sɐndʱi], "joining") is a cover term for a wide variety of sound changes that occur at morpheme or word boundaries. Examples include fusion of sounds across word boundaries and the alteration of one sound depending on nearby sounds or the grammatical function of the adjacent words. Sandhi belongs to morphophonology.
Sandhi occurs in many languages, particularly in the phonology of Indian languages (especially Sanskrit, Tamil, Sinhala, Telugu, Marathi, Hindi, Pali, Kannada, Bengali, Assamese, Malayalam). Many dialects of British English show linking and intrusive R.
It may be extremely common in speech, but sandhi (especially external) is typically ignored in spelling, as is the case in English (exceptions: the distinction between a and an; the prefixes syn-, in-, en-, and con-). Sandhi is, however, reflected in the orthography of Sanskrit, Sinhala, Telugu, Marathi, Pali and some other Indian languages, as with Italian in the case of compound words with lexicalised syntactic gemination.
Most tonal languages have tone sandhi in which the tones of words alter according to certain rules. An example is the behavior of Mandarin Chinese; in isolation, tone 3 is often pronounced as a falling-rising tone. When a tone 3 occurs before another tone 3, however, it changes into tone 2 (a rising tone), and when it occurs before any of the other tones, it is pronounced as a low falling tone with no rise at the end.
An example occurs in the common greeting 你好 nǐ hǎo (with two words containing underlying tone 3), which is normally pronounced ní hǎo. The first word is pronounced with tone 2, but the second is unaffected.
The French liaison, in which a word-final consonant that is normally silent when occurring at the end of a phrase or before another consonant, is pronounced as if part of the next word when followed by a vowel, can be considered a form of external sandhi. For example, deux frères (two brothers) is pronounced /dø fʁɛʁ/ with a silent ⟨x⟩, and quatre hommes (four men) is pronounced /katʁ ɔm/, but deux hommes (two men) is pronounced /døz‿ɔm/.
In Japanese phonology, sandhi is primarily exhibited in rendaku (consonant mutation from unvoiced to voiced when not word-initial, in some contexts) and conversion of つ or く (tsu, ku) to a geminate consonant (orthographically, the sokuon っ), both of which are reflected in spelling – indeed, the っ symbol for gemination is morphosyntactically derived from つ, and voicing is indicated by adding two dots as in か／が ka, ga, making the relation clear. It also occurs much less often in renjō (連声), where, most commonly, a terminal /n/ on one morpheme results in an /n/ (or /m/) being added to the start of the next morpheme, as in 天皇: てん ＋ おう → てんのう (ten + ō = tennō); that is also shown in the spelling (the kanji do not change, but the kana, which specify pronunciation, change).