Dzelo stands eighth, writes the affricate dz, and carries the numeral 6. Its name is the Slavonic word meaning very or greatly.
Its position in the number series is peculiar and instructive. Greek had lost its letter for 6 long before the ninth century and used a separate numeral sign in that slot, so there was no Greek letter for Cyrillic to borrow. Dzelo therefore occupies a numeral position without inheriting a shape to go with it, and its sign is generally taken from the Glagolitic letter of the same name. It is the only letter in the alphabet where the Greek series supplied a number and no letter.
The sound had a short life. In most of the East Slavic area dz fell together with plain z within a few centuries, leaving two letters where the language had one sound. The letter did not disappear when its sound did, because it was still needed as the numeral 6, and this is a pattern worth noticing: the numeral function repeatedly kept letters alive long after the phonology had finished with them.
One modern descendant kept the sound as well as the sign. Macedonian has a letter for dz in its alphabet today, writing a sound its neighbours lost, so what reads in a Russian context as an obsolete liturgical curiosity is in Skopje an ordinary working letter.