Normally, the transition probability from the non-emitting entry state to the non-emitting exit state of a HMM will be zero to ensure that the HMM aligns with at least one observation vector. Models which have a non-zero entry to exit transition probability are referred to as tee-models.
Tee-models are useful for modelling optional transient effects such as short pauses and noise bursts, particularly between words.
Although most HTK tools support tee-models, they are incompatible with those that work with isolated models such as HINIT and HREST. When a tee-model is loaded into one of these tools, its entry to exit transition probability is reset to zero and the first row of its transition matrix is renormalised.