For rule-based scheme, the parametric form needs to be compromised with the easiness of finding coarticulation rules for parameters. Hence, speech models that are more closely physiology based, such as formant synthesizer and articulatory synthesizer, were used for rule-based scheme. The rule-based scheme relies on good rules to describe the coarticulations. These rules are derived from detailed analysis of the speech database. For example, formant transitions are describe by the loci theory in formant synthesis.
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.