Rules become complicated in order to achieve certain degree of naturalness. Since the synthesis model in rule-based scheme tends more physical, it is in principle more easy to implement new speaker or different voice qualities than concatenative scheme. The coarticulation information are stored within these units. There are approximate 10,000 syllables and 2000 diphones in English. To trade off between the number of units required for storage and the amount of coarticulation information, diphone is often chosen as the speech unit.
Some researchers also investigate on non-uniform units.
Second is the concatenation of chosen units. The concatenation is often a simple linear interpolation. To reduce both the coarticulatory effect and segment mismatches at unit boundaries, methods of incorporating contextual information including clustering techniques are used in the unit selection procedure. 19-31, Feb. 1992. The acoustic realization for a specific phoneme may not be the same for different words or under different context.