Based on the production and acceptability data of tone sandhi patterns of the Huai’an dialect of Jianghuai Mandarin (Huai’an, hereafter) at the syntactic level, this paper argues that (i) both disyllabic and trisyllabic tone sandhi domains are basic domains in Huai’an, which differs from the traditional analysis where only disyllabic domain exists in the Mandarin language family and trisyllabic domain is derived, and (ii) as a consequence, both types of domains can be built freely at the post-lexical level as long as all the syllables are exhaustively incorporated. By positing both disyllabic and trisyllabic domains, the current proposal gives a succinct analysis of Tone 3 Sandhi in Huai’an, and removes the parameter of directionality. Furthermore, a lapse-based analysis (Elenbaas & Kager, 1999) involving a binary tone sandhi domain with an unparsed syllable is unlikely because it cannot produce the surface representation “(2 2 3)” (parenthesis indicates tone sandhi domain boundary and number indicates tone) where all three syllables must be included into one tone sandhi domain to generate the correct surface form in Huai’an. This study suggests more generally that ternary prosodic units, including ternary stress feet (Prince, 1980), can be independent domains in phonology.