Sandhi

Sandhi (Sanskrit: सन्धि, "joining") is a phonological process where adjacent sounds at word or morpheme boundaries modify each other — the Sanskrit term has been adopted into Western linguistics as the general name for this process across all languages. Sandhi appears in most Indian languages (Sanskrit, Tamil, Malayalam, Hindi, Bengali, Marathi) and is typically classified as either external sandhi (applying across word boundaries — "car park" → "carpark" pronunciation in English is a mild example) or internal sandhi (applying within a word at morpheme boundaries). In Sanskrit and Tamil, sandhi is rigorously systematised with specific phonetic rules — a vowel + vowel combination, a consonant + consonant combination, or a consonant + vowel combination each triggers a predictable transformation. Sandhi is one of the features that makes Indian-language TTS difficult: a system that treats each word in isolation will mispronounce content where two words trigger sandhi. Good Indian-language TTS applies sandhi rules during text normalisation, producing natural-sounding speech at word junctions.

How it works

In Sanskrit, sandhi is so systematic that it's essentially a set of algebraic rules: /a/ + /a/ = /aː/ (long a), /a/ + /i/ = /eː/, /i/ + /a/ = /ya/, etc. Students of classical Sanskrit memorise hundreds of sandhi rules. Tamil sandhi includes glide insertion (/i/ + vowel → /i/ + y + vowel), consonant assimilation, and context-dependent voicing changes. Malayalam sandhi is particularly complex because it's explicitly written out — whereas Sanskrit sandhi often appears in the pronounced form only, Malayalam spells out the result of sandhi in the written script, producing the conjunct-heavy appearance that makes Malayalam text rendering hard. Modern colloquial Hindi applies sandhi less rigidly but still respects common patterns — "अधिक + आर्थिक" → "अधिकार्थिक" blurs the word boundary audibly. TTS systems that apply sandhi at text-normalisation time produce smoother, more natural audio compared to systems that pronounce each word as if isolated.

Examples

Related terms

Learn more

Frequently Asked Questions