Slovo stands nineteenth, writes s, and carries the numeral 200 from Greek sigma. Its name is the Slavonic noun for word, and of all the letter names in the alphabet it is the one that carries the most weight, because it is the word the Slavonic translators chose to render the Greek term at the opening of the fourth Gospel.
The shape is the lunate sigma, the crescent form of the Greek letter used in the uncial books, rather than the angular sigma of classical inscriptions. That choice is another small dating fact: it fixes the model as the Byzantine liturgical hand and not any earlier Greek writing.
As a numeral it is 200. In the church books it is also one of the letters most often abbreviated under a titlo, because it begins a large number of the recurring words of the services.
The letter came through to the modern alphabets without change of shape or sound. Its name did not survive in ordinary use, but it survives in one place a Slavonic reader still meets: the numbering and titling of the divisions of certain older books, where a section is called a slovo, a word or discourse, in the same sense the Greek tradition used for a homily.
The name has one further resonance and it is contested. The Slavic self designation has long been derived from this same word, on the reading that the people who share a language are the people of the word, set against the older Slavonic term for a foreigner which is built on the idea of being unable to speak. The derivation is old, widely repeated and not settled; other origins have been proposed and the question belongs to comparative etymology rather than to the history of the alphabet. What is certain is that a Slavonic reader met this letter in a word carrying more weight than any other in the set.