Dobro stands fifth, writes d, and carries the numeral 4 from Greek delta. Its name is the Slavonic noun and adjective for good, one of the commonest words in the language and in the liturgy that the alphabet was built to carry.
The shape is the uncial delta, a triangle standing on a base with small feet, and the feet are worth noticing because they are one of the features that make an early Cyrillic page look Greek at a distance. The hand the letters were fitted to was the formal Byzantine book hand, upright, evenly spaced, with each letter drawn separately rather than joined, and dobro sits in that hand without adjustment.
In the numeral series it does the work of 4, which puts it on the fourth Gospel, the fourth ode, the fourth tone of the eight tone cycle and the fourth of the great fasts, all of which are numbered constantly in liturgical books. A reader of those books meets the letter as a number at least as often as a sound.
It survives in every modern Cyrillic alphabet, and its printed form is one of the few that changed noticeably: the civil type cut in the early eighteenth century gave it the shape now familiar, while the church books kept the older triangular letter. Both forms are still in use, in different kinds of book, for the same sound.