Tvrido writes the consonant t and takes the numeral three hundred. Its name is an adverb, firmly or hard, and it closes the clause that the preceding two names begin.
The sound is a plain dental stop. Like the other stops it is stable across Slavic and offers no evidence for dating a text, but it is very common, and its treatment in a hand is one of the features by which a scribe is recognised.
The numeral three hundred comes from counting the Glagolitic letters in their own sequence, this being the twenty first letter. It coincides with the Cyrillic value, as the neighbouring numbers do, for reasons of arithmetic rather than of design.
The name is also the point at which the recitation of the alphabet, read as a text, reaches something like a conclusion. Whatever is made of the names that follow, the first twenty produce Slavonic clauses that most readers accept, and they end here. That the first twenty do so and the rest do not is itself an observation that any account of the names has to deal with, and the two obvious explanations, that the composer ran out of suitable words and that the later names were inherited from elsewhere, are both available and neither is demonstrated.