Djervi writes a palatal consonant and carries the numeral thirty. It is one of the Glagolitic letters that has no counterpart in the early Cyrillic alphabet at all, and its presence is often cited as evidence about where the script was actually put to use, since the sound it records is not one that the Slavonic of the canon needs frequently.
The letter appears in the earliest manuscripts chiefly in foreign words and names, where it renders a sound the surrounding language did not have a plain sign for, and in a small number of native words in which a consonant had been softened. Its rarity means that a manuscript may go many folios without using it, and that its form is less standardised across the tradition than the forms of the common letters.
In the numeral system it is the figure thirty, on the Glagolitic principle of counting the alphabet through in its own order.
The sound the letter records became important later and further west. The South Slavic languages developed palatal consonants of their own, and when Cyrillic was adapted to write Serbian in the nineteenth century, signs had to be devised for them. That is a separate development nine hundred years after the fact, and the Glagolitic letter is its ancestor only in the loose sense that both answer to a similar need.