Sandhi

Sandhi ( "" , "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 Tamil, Sanskrit, Sinhala, Telugu, Marathi, Hindi, Pali, Kannada, Bengali, Assamese, Malayalam). Many dialects of British English show linking and intrusive R.
A subset of sandhi called tone sandhi more specifically refers to tone changes between words and syllables. This is a common feature of many tonal languages such as Mandarin Chinese.
Types.
Internal and external sandhi.
Sandhi can be either
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.
External sandhi effects can sometimes become morphologised (apply only in certain morphological and syntactic environments) as in Tamil and, over time, turn into consonant mutations.
Tone sandhi.
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 "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.
Examples.
French.
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, ' (two brothers) is pronounced with a silent , and ' (four men) is pronounced , but "" (two men) is pronounced .
Japanese.
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 or ("tsu", "ku") to a geminate consonant (orthographically, the sokuon ), both of which are reflected in spelling – indeed, the symbol for gemination is morphosyntactically derived from , and voicing is indicated by adding two dots as in "ka", "ga", making the relation clear. It also occurs much less often in , where, most commonly, a terminal on one morpheme results in an (or ) being added to the start of the next morpheme, as in ("ten" + "ō" = "tennō"); that is also shown in the spelling (the kanji do not change, but the kana, which specify pronunciation, change).